



Class 

Book_ 

Copyright N° 



COPYRIGHT DEPOSIT. 



MECHANICAL DRAWING 



MECHANICAL 
DRAWING 



A TREATISE ON THE DRAWING OF MECH- 
ANISMS AND MACHINE DETAILS, INCLUD- 
ING- THE MAKING OF DIFFERENT CLASSES 
OF DRAWINGS, THE DIMENSIONING, READ- 
ING, AND CHECKING OF WORKING DRAW- 
INGS, NUMBERING AND FILING SYSTEMS 
FOR DRAWINGS, AND GENERAL DRAFTING 
ROOM PRACTICE 



BY 

FRANKLIN D. JONES 

Associate Editor of MACHINERY 

Author of " Thread-cutting Methods," " Mechanisms 

and Mechanical Movements," "Turning and Boring," 

" Planing and Milling." etc. 



FIRST EDITION 

FIRST PRINTING 



NEW YORK 

THE INDUSTRIAL PRESS 

London: THE MACHINERY PUBLISHING CO., Ltd. 
I920 



Ti53 
i °i &o 



Copyright, 1920, 

BY 

THE INDUSTRIAL PRESS 
NEW YORK 



SEP 30 lb2U 



COMPOSITION AND ELECTROTYPING BY THE PLIMPTON PRESS, NORWOOD, MASS. 

©CI.A597584 



PREFACE 



As mechanical drawing has been a very popular subject 
among students in schools and shops, numerous text-books 
have been published on mechanical drafting practice. This 
book is added to the list because the publishers believe that 
there has been need for a treatise dealing more thoroughly 
with methods which are actually employed in well-managed 
drafting-rooms. Many books on mechanical drawing have 
covered such subjects as geometrical drawing problems, ortho- 
graphic projection, the development of intersecting surfaces, 
etc., but the application of these principles and the real object 
of mechanical drawing as related to machine and tool manu- 
facture has been dealt with vaguely, in many instances. The 
student has been taught certain details, but he has not been 
given a clear conception of the work of draftsmen and de- 
signers in the drafting-rooms of machine-building plants. 
This book presents the subject in a way that will enable the 
student to understand what the term "mechanical drawing" 
really means in its broadest sense, the essential features of 
modern drafting practice, and the difference between the 
mere representation of a design by a suitable drawing and 
the more valuable work of originating and developing the 
design itself. 

A special effort has been made to secure a well-balanced 
treatise in which the various elements of mechanical drawing 
are dealt with according to their relative importance. For 
instance, little space is given to lettering, because making 
fancy letters in numerous styles is not the work of a drafts- 
man in a well-managed drafting-room, although this subject 
has been greatly emphasized in many books. The aim has 
been to present methods which are in actual use rather than 
exercises in drawing which do not conform to the practice in 



VI PREFACE 

manufacturing plants. An elaborate drawing of a bevel 
gear with all of the teeth accurately reproduced may be an 
attractive and impressive feature in a text-book on mechani- 
cal drawing, but it is misleading, because working drawings 
are not made in that way. This book, in its arrangement 
and scope, is based on the assumption that it is essential for 
the student of mechanical drawing — whether in school or 
in shop — to understand the purpose of drawings as applied 
to machine and tool construction, how various mechanical 
devices may be represented by means of drawings, the neces- 
sity of making drawings which completely and clearly show 
what they are supposed to, and the relation between drawing 
and designing. Special attention has been given to the di- 
mensioning of drawings and to the importance of using printed 
instructions or any legitimate means of making a drawing 
entirely clear to the men in the shop. 

In dealing with the numerous details of the draftsman's 
work, an effort has been made to present methods which are 
sanctioned by common usage and to explain the reasons for 
the more important variations in practice. To accomplish 
this the methods and systems of many of the representative 
drafting-rooms were studied, and much valuable information 
was also secured from articles pertaining to different features 
of drafting practice which have been published in Machinery 

F D T 

New York, September, 1920. * J ' 



CONTENTS 



Chapter I 

DRAWINGS AND THEIR USE IN MACHINE AND 
TOOL CONSTRUCTION 

PAGES 

General Uses of Drawings — The Work of the Draftsman 
— Why Draftsmen Should Understand Manufacturing Methods 1-9 

Chapter II 
PROJECTION AS APPLIED TO MECHANICAL DRAWING 

Orthographic Projection — Number and Arrangement of 
Views — Examples of Projection Drawings 10-26 

Chapter III 

MECHANICAL DRAWING INSTRUMENTS AND 
MATERIALS 

Instruments for Drawing Straight Lines and Circles — 
T-Square and Triangles — Protractors — Scales — Drawing- 
boards — Paper and Other Materials 27-69 

Chapter IV 

HOW DESIGNS ARE ORIGINATED AND PROCEDURE 
IN MAKING DRAWINGS 

Preliminary Work in Designing — Different Classes of 
Drawings — Scale Drawings — Kinds of Lines Used — 
Tracing 7°~97 

Chapter V 
SECTIONAL VIEWS AND THE READING OF DRAWINGS 

Examples of Sectional Views — Use of Section Lines — Prin- 
ciples Governing Reading of Mechanical Drawings 98-129 



viii CONTENTS 

Chapter VI 
METHODS OF DIMENSIONING WORKING DRAWINGS 

Rules for Dimensioning — Designating Tolerances on Draw- 
ings — Designating Angles and Tapers — Tabulated Drawings 130-1 67 

Chapter VII 

INSTRUCTIONS ON WORKING DRAWINGS AND 
PROCEDURE WHEN CHECKING 

Use of Symbols and Abbreviations — Explanatory Notes 

— Titles — Lettering — Checking Drawings Systematically 168-181 

Chapter VIII 

PRINTING PROCESSES AND APPARATUS FOR 
PRINTING, WASHING AND DRYING 

Making Prints for Shop Use — Methods of Drying and Iron- 
ing Prints — Changing Blueprints — Mounting Blueprints. . 182-196 

Chapter IX 

ENGINEERING STANDARDS AND DRAWINGS OF 
MACHINE DETAILS 

Screw Threads and Their Standards — Conventional 
Methods of Representing Screw Threads — Pipe Fittings — 
Keys — ■ Gear Drawings 197-234 

Chapter X 
DESIGNING OR LAYING OUT CAMS 

Laying Out Cams for Uniform Motion, Intermittent Mo- 
tion, Crank or Harmonic Motion, and Uniformly Accelerated 
Motion — Different Types of Cams 235-256 

Chapter XI 

GEOMETRICAL DRAWING PROBLEMS AND THE 
DEVELOPMENT OF INTERSECTING SURFACES 

The Ellipse, the Helix, Involute and Cycloidal Curves 

— Sheet Metal Pattern Drawing 257-272 



CONTENTS i x 

Chapter XII 

DRAFTING-ROOM SYSTEMS, EQUIPMENT AND 
ARRANGEMENT 

Filing Systems — Card Indexes — Recording Changes — 
Lists of Parts — Arrangement of Drafting-room and Its 
Lighting 273-309 

Chapter XIII 
SKETCHING AND PERSPECTIVE DRAWING 

Making Sketches without the Use of Instruments — How 
Perspective Drawings are Made — Isometric Drawing 310-337 



MECHANICAL DRAWING 



CHAPTER I 

DRAWINGS AND THEIR USE IN MACHINE AND TOOL 
CONSTRUCTION 

When any new or improved form of tool or machine is 
being developed, its general arrangement and the principle 
governing its operation or use may be quite clear in the mind 
of the inventor or originator, and he may proceed with the 
actual work of construction, guided only by a mental picture 
of the device. Many - simple tools or appliances could be, and 
some are, produced in this direct way, but it is evident that 
such a method of procedure is greatly restricted. It is often 
easier for the originator of a new type of mechanism to build 
it with his own hands than to attempt, simply by verbal 
description, to give some one else a clear enough mental pic- 
ture of the device to enable him to construct it. This direct 
method of construction is, of course, impracticable as applied 
to regular manufacturing. In the first place, it would be 
impossible to originate many of the more complicated mecha- 
nisms by simply forming a mental picture of them. The basic 
principle of the device and possibly its general arrangement 
might be entirely clear, but in order to determine the exact 
relation of the various parts when they are all properly pro- 
portioned and assembled, it is necessary to make a fairly 
accurate drawing. Such a drawing not only shows the ar- 
rangement of the mechanism as a whole, but greatly assists 
the designer, in many cases, in the development of the idea. 
Frequently the mental picture is distorted and, when an accu- 
rate drawing is made, it is apparent that changes are neces- 
sary either in the form and size of one or more parts or possibly 
in the entire arrangement of the mechanism. 



2 MECHANICAL DRAWING 

General Uses of Drawings. — The method usually fol- 
lowed by . inventors and designers in originating new or im- 
proved mechanical appliances is to make a drawing of what- 
ever plan or idea is to be developed. When this has been 
done, a clear conception of the form and often of the prac- 
ticability of the device represented by the drawing may be 
obtained not only by the originator of the idea but by others 
who understand drawings and are able to "read" them. 
Drawings, then, as applied to the manufacture of tools and 
machines, serve several important purposes. First, they 
assist in the development of a plan by enabling the inventor 
or designer to see clearly the relation of different parts to one 
another and whether or not the desired motion or effect may 
be obtained. Second, drawings make it possible for the 
originator of a plan to convey the idea to others readily. 
Third, they show to those who are actually to construct the 
device, the proportions of its different parts and their rela- 
tion when properly assembled. Finally, drawings are useful 
as records of what has been done and make it possible to re- 
produce whatever tool or mechanism is represented on the 
drawing. 

The Work of the Draftsman. — In the manufacture of vari- 
ous kinds of mechanical tools and equipment, the work to be 
performed may be divided into four branches: (i) Originating 
entirely or in part the general type of device to be constructed 
and the principle governing its operation; (2) designing the 
mechanism in accordance with established mechanical prin- 
ciples and in such a way that the different parts will be strong 
enough to resist any stresses to which they may be subjected; 
(3) making drawings such as are needed in the actual work of 
construction; and, (4) making, fitting, and assembling the 
different parts. In the study of mechanical drawing, it is 
essential to understand the relation of these four branches of 
work to one another, because a draftsman may simply make 
drawings according to the ideas of others, or he may have more 
or less to do with originating the plan. Some draftsmen are 
also able to determine the proportions of different parts and 



USE OF DRAWINGS 3 

many of them control, to some extent, the method of manu- 
facturing. In a restricted sense, a draftsman makes drawings 
of appliances originated by an inventor or designer. The 
designer may be an inventor or vice versa, and he is always 
a draftsman and is capable of making mechanical drawings. 
The draftsman, however, is not necessarily a designer and 
may know little or nothing about the principles governing 
the design of machinery or tools. 

It is evident, then, that a man is valuable in the designing 
department in proportion to his ability to originate, design, 
and develop useful and practical appliances. It is also ap- 
parent that the name " draftsman" has a broad meaning and 
may include anyone, from a man who can make a drawing 
to scale from a free-hand sketch to a man who can design as 
well as draw a complicated automatic machine. The first 
one is a draftsman pure and simple, while the second one is a 
designing draftsman," who can create. Properly designated, 
the first man is a draftsman, while the second is a designer. 
This distinction, however, is not usually made except in 
salary, and anyone working on drawings (not tracings) is 
known by the general term of draftsman. If the work is 
restricted to the making of tracings, the one doing it is com- 
monly known as a tracer. A designer must be a specialist, 
because it is impossible for one man to know how to design 
mechanical devices for any and all purposes, and there is no 
known rational method of design which can be studied in 
the same way as one might study mathematics or physics. 

What the Draftsman Should Know. — In taking up the 
study of mechanical drawing, it is important to know what 
is involved in becoming a designer or draftsman who, instead 
of simply making drawings of the plans of others, is capable 
of original work. To begin with, the ability to originate or 
improve plans and designs may be developed by studying 
what others have accomplished. It frequently happens that 
the principle governing the operation of one device may be 
applied to some other mechanism which is used for an entirely 
different purpose. In this way, the original idea is made more 



4 MECHANICAL DRAWING 

useful and of greater value because it is utilized for more than 
one purpose. While this is not original work in the exact 
meaning of the term, the fact is that very few mechanical 
appliances are absolutely originated by one man; moreover, 
it may not always be advisable in machine design to attempt 
to be entirely original, but rather to apply what is definitely 
known to be sound in theory and practice. This does not 
mean that the inventor or designer should not think for him- 
self nor that he should deliberately appropriate the ideas of 
others, but simply that one should proceed cautiously when 
attempting to improve or change entirely some commonly 
accepted method or principle which has been thoroughly 
tested in practice. 

The draftsman whose work is not confined merely to draw- 
ing lines on paper, must have a knowledge of mechanical 
laws, the various well-known methods of transmitting and 
modifying motion, and how to proportion parts of tools and 
machines so that they will resist the stresses to which they 
are subjected. Many worthless designs have been the direct 
result of ignorance of fundamental mechanical principles. 
Another requirement is a knowledge of the art of drawing, 
which is the principal subject dealt with in this book. While 
a draftsman, to be successful, must know more than how to 
make mechanical drawings, nevertheless this is an important 
part of his work, because drawings which do not clearly repre- 
sent the object drawn are a source of trouble and are liable 
to cause serious mistakes. Delays in the pattern shop and 
machine shop are often due to poor drawings which are lack- 
ing either in dimensions, in the arrangement and number of 
the views, or in some other respect. 

Why Draftsmen Should Understand Manufacturing 
Methods. — A fourth requirement in connection with the 
work of designing is a knowledge of manufacturing methods. 
Other things being equal, the draftsman excels who is capable 
of designing parts which are as simple and free from com- 
plications as possible and which are, therefore, cheaper to 
manufacture. The competent designer not only thinks of 



USE OF DRAWINGS 5 

the operation of a tool or mechanism, but carefully considers 
the work of the patternmaker, molder, machinist, and tool- 
maker. It is much easier/of course, to draw lines on paper 
than to form the parts which the lines represent, in wood, 
iron, and steel; yet this simple fact is often disregarded and 
many designs and inventions have been discarded because 
the cost of manufacturing was unnecessarily high. Since 
a simple change on a drawing will often greatly reduce the 
work in the pattern shop, the designer should understand the 
principles of patternmaking and molding. A knowledge of 
machine shop practice is even more important, and for this 
reason experienced machinists and toolmakers who take up 
drafting work find that their shop training is invaluable, 
especially when designing tools, jigs, or parts which require 
considerable machine work. The draftsman who has not had 
acutal experience in the shop should consult with machinists, 
patternmakers or others who may be able to supply valuable 
information. When making special tools, such as jigs, milling 
fixtures, etc., for reducing the cost of the WT>rk ; the foreman 
or workman should not only be consulted, but should usually 
have a deciding voice as to what should be made. The men 
who use the tools often know better than the draftsman what 
is needed.' 

The machinist and shop foreman, being constantly w r ith 
the work, know which are the expensive operations, when 
there is difficulty in fitting, where the clearances are so small 
as often to become interferences, and other facts of import- 
ance in developing a good design. A method that has been 
used with excellent results is for each foreman and responsible 
workman to have a blank stub-book with the pages numbered 
and provided with suitable printed headings. In this book 
all suggestions are written and the perforated leaves are removed 
and sent to the chief draftsman, the stub being kept as a 
memorandum for the shop man. These leaves are sorted 
when received, and those requiring immediate attention are 
investigated; others are filed under the respective machines 
until another lot is to be built, when the suggestions are con- 



6 MECHANICAL DRAWING 

sidered collectively. The workmen are not only at liberty 
to use these books, but are held accountable if they allow 
troubles to exist on the machines they build, and do not re- 
port them. In this way, advantage is taken of the mechanical 
knowledge and the ideas stored up in the minds of the men 
who are actually doing the work. 

Mechanical Drawings and Their Application. — A mechani- 
cal drawing is a representation on paper of a machine, ma- 
chine part, tool, or other object used in the mechanical indus- 
tries, that is to be made from metal, wood, or other material, 
and it may show either the form or shape to which a machine 
part is to be made, or the relation of a number of parts to 
each other in order to indicate how these parts are to be as- 
sembled; hence, a mechanical drawing may be used to repre- 
sent anything from a locomotive to a small machine screw. 
These drawings are generally made on a drawing-board by 
means of a T-square, triangles, compasses, scales, etc., but 
sometimes they are made without the use of any instruments 
except a pen or pencil, in which case they are generally known 
as " free-hand drawings" or "sketches." All instruments, 
appliances, and accessories necessary for the production of a' 
mechanical drawing are generally known by the general term 
"drawing instruments," except the various kinds of paper, 
cloth, etc., upon which the drawing is made, and the pencils, 
ink, and erasers, which are grouped under the head of "draw- 
ing materials." 

Classification of Mechanical Drawings. — Mechanical draw- 
ings may be classified under two main headings, outline draw- 
ings and working drawings. Outline drawings merely show the 
general appearance and over-all dimensions of machines and 
devices and are used mainly in catalogues and for represent- 
ing the general features of a machine to prospective pur- 
chasers. Working drawings are used in machine shops and 
pattern shops in the building of machines and tools. Working 
drawings may be of two kinds. The first class, known as 
general or assembly drawings, show all the parts of a machine 
or mechanism, in their proper position and relation to each 



USE OF DRAWINGS 7 

other. The principal dimensions may or may not be given 
on drawings of this kind, which are used by the assemblers and 
erectors in the fitting and assembling of machine parts that 
have already been made in other departments of the shop. 
The second class of working drawings, known as detail draw- 
ings, give all the dimensions and complete information as to 
the form of separate machine parts or of sections composed 
of several parts. The dimensions on working drawings in- 
dicate the sizes of all parts requiring machining operations, 
and such drawings should also contain complete information 
regarding the material from which parts are to be made and 
the treatment they are to undergo — such as hardening, case- 
hardening, etc. The assembly drawings are generally made 
to a much smaller size or scale than that of the actual ma- 
chine or tool, while the detail drawings are made either full 
size, or to as large a scale as possible. 

A working drawing must convey to the eye of the work- 
man a clear idea of what the designer wants made. It should 
be so complete that, when it passes into the shop, no further 
questions or explanations will be necessary; hence, a com- 
plete working drawing contains all the necessary information 
as to materials, treatment, limits, fits, finish, etc., that the 
shop man requires. 

Mechanical drawings do not represent an object in the form 
of a picture — that is, they do not show the object in the 
same way as it would appear to the eye of the observer. A 
drawing made to appear exactly as it would when viewed 
from a certain point is known as a perspective drawing, and 
the mechanical draftsman is seldom required to represent 
machines or machine parts in this way. He conveys his 
ideas by much simpler and better methods than this and, in 
making working drawings, uses what is known as orthographic 
projection, or simply projection. Many mechanical drawings 
do not appear to the untrained eye to represent the true form 
of an object — that is, they do not always look like the object, 
as would a picture for instance, — because certain methods 
of representing machine parts have been adopted, by means 



8 MECHANICAL DRAWING 

of which drawings can be made much more rapidly and the 
exact form and dimensions can be indicated more accurately 
than if the true perspective form were reproduced. As an 
example, screw threads are not ordinarily drawn in the way 
in which they actually appear to the eye, but a much easier 
and quicker way of representing them is used. 

General Views and Detail Drawings. — When it is desired 
to make a drawing of a machine already constructed, each 
part is measured and sketched separately, and all necessary 
dimensions are placed upon the sketch; then these parts are 
assembled, so to speak, in the form of a general drawing. 
On the other hand, if it is desired to design a machine, 
a general drawing with the parts in place is made, and then 
the dimensions of the various parts are determined and the 
extent to which they must be machined by turning, planing, 
milling, drilling, reaming, tapping, etc. The different parts 
or small units consisting of several parts, or of a great many 
parts in some cases, are drawn separately, at least in suffi- 
cient detail to show clearly what is wanted. 

In the general views, outlines are drawn of such details as 
are thought essential to clearness. If the machine, tool, or 
other device is not complicated and consists of a relatively 
small number of parts, a general drawing may be sufficient, 
but if there are a great many separate parts, separate working 
drawings of these details are necessary. If all the details of 
a complicated design were drawn on the general view, there 
would be so many lines that it would be difficult, if not im- 
possible, to show clearly the form and size of each part. The 
smaller detail drawings are also much more convenient for 
shop use. 

A mechanical drawing should show clearly the form and 
dimensions of the part it represents. Every line should 
stand for some definite thing; and when lines cannot express 
the ideas in a direct and unmistakable manner, abbrevia- 
tions, symbols, and printed notes should be used so that the 
patternmakers or machinists who are to use the drawings 
will be able to work without other instructions. Each note 



USE OF DRAWINGS 9 

should consist of concise sentences and should be placed close 
to the part to which it refers, in order that it may be easily 
read and understood. The different views should be ar- 
ranged so that they clearly represent the object drawn, and a 
reasonable degree of neatness in the drawing of lines and in 
lettering is also desirable. 

Why a Knowledge of Mechanical Drawing is Essential. — 
Every man engaged in the mechanical trades who has aspira- 
tions toward advancement must, at least, learn how to "read" 
or understand mechanical drawings; and in order to obtain 
a complete knowledge of the reading of mechanical drawings, 
it is necessary to know, in a general way, how to make them. 
A man who cannot make accurate and understandable 
sketches according to the methods of mechanical drawing is 
seldom able to read any except the simplest drawings; hence, 
the importance of the study of mechanical drawing. Draw- 
ing may be called a universal language, and the mechanic 
who cannot read drawings or blueprints is handicapped in his 
trade almost as much as if he could not read or write. The 
knowledge of how to make sketches and drawings is also an 
exceedingly useful accomplishment to a man who, as foreman 
or superintendent, has to direct the activities of others. The 
importance of understanding the principles of mechanical 
drawing, therefore, is apparent and it should be clearly under- 
stood that not only draftsmen must understand these prin- 
ciples, but every mechanic who wants to read drawings rapidly 
and accurately. 

In order to read a working drawing, it is necessary that 
one be familiar with the conventional methods commonly 
used to represent parts, material, finish, etc., and that one 
understand in what respects mechanical drawings differ from 
perspective drawings or photographs, which represent the 
object as it appears to the eye. To the inexperienced, a 
working drawing may appear like a conglomeration of lines 
which do not represent clearly what they are intended to 
show, but the man who understands such drawings will have 
a mental picture of the object drawn. 



CHAPTER II 
PROJECTION AS APPLIED TO MECHANICAL DRAWING 

The mechanical drawing of a machine part or a combi- 
nation of parts forming a complete mechanism is usually 
composed of two or more separate views, each representing a 
different side of the object drawn. The number of separate 
views on the drawing depends upon the number actually 
needed to show clearly the general shape of the piece and all 
important dimensions such as the lengths and widths of dif- 
ferent parts, the shape as seen from different sides, and, in 
brief, whatever information concerning the form and dimen- 
sions is needed for reproducing the mechanical device which 
the drawing represents. 

Perspective drawings which show objects just as they 
appear to the eye are useful, especially when the idea is 
merely to show the general shape of whatever part is repre- 
sented. The use of such drawings in books and periodicals as 
a means of illustrating various objects is, of course, common. 
In mechanical work, and especially wherever machinery is 
constructed, drawings are used which are quite different in 
appearance from the ordinary perspective drawings. A per- 
spective drawing may show two or three sides of an object 
on one view, the same as a photograph. While a mechanical 
drawing does not resemble a perspective drawing in appear- 
ance, it does show the form of the object and usually much 
more clearly and accurately than a perspective drawing, pro- 
vided the mechanical drawing is understood. 

Orthographic Projection. — Mechanical drawings are based 
on a method of drawing known as " orthographic projection." 
In order to illustrate simply this method of drawing, assume 
that some object is held in the hand on the same level as the 
eyes and is turned so that the front side, top side, and end 



PROJECTION 



II 



are each seen successively. These different views will then 
correspond practically to the different views of the same 
object as represented by a mechanical drawing made accord- 
ing to the orthographic projection method. In other words, 
if three sketches or separate views were made, showing the 
outline of the object as it appeared when seen from the three 
different positions, such views would correspond to those on a 
mechanical drawing of the same piece. While these different 
views would all be drawn on a flat sheet or on a plane surface, 
they are practically the same as those that would be obtained 
if the object drawn were held in the hand and were turned 



SIDE 
VIEW 



Machinery 



Fig. 1. (A) Perspective Drawing of a Rectangular Block, 
of a Rectangular Block 



(B) Mechanical Drawing 



first to one position and then the other, in order to show the 
different sides as just described, except that many mechanical 
drawings are arranged to show parts and shapes that would 
be concealed when actually looking at the object itself. 

These mechanical drawings, which are composed of views 
representing different sides of a machine part, tool, or some 
other mechanical device, show the length, breadth, and thick- 
ness of various portions of the piece accurately, which is a 
great advantage in mechanical work, because the chief pur- 
pose of most mechanical drawings is to represent mechanical 
devices so clearly and accurately that they may readily be 



12 MECHANICAL DRAWING 

reproduced in iron and steel. It must not be inferred from 
this that drawings are made so accurately that the pattern- 
maker, machinist, and toolmaker can measure them in order 
to determine the required dimensions at various places. This 
method of determining the dimensions is unnecessary, because 
an important part of the draftsman's work is to place on the 
drawing all necessary dimensions expressed either in feet, 
inches, or fractional parts of an inch, depending upon the 
size of the work and the degree of accuracy required. 

Comparison of Perspective and Mechanical Drawings. — 
The relation between an ordinary perspective drawing and a 
mechanical drawing made according to the orthographic pro- 
jection method, is illustrated in Fig. i, which shows, at A, a 
perspective view of a plain rectangular block, and at B, a 
mechanical drawing consisting of front, plan, and side views. 
The side view is practically the same as though the right- 
hand side of the block were removed and turned around so 
as to be in line with the front side. Similarly, the plan or 
top view represents the upper side of the block, as though 
it were swung upward to a vertical position and in the same 
plane as the front and side views. The front view shows that 
this side of the block is square, but by simply referring to 
this view alone, it is impossible to determine whether or not 
the block is a cube with sides of equal width, or a block of 
rectangular form. The side and plan views show, at a glance, 
that the block is not a cube and that its sides are of rectangular 
shape. Each view represents the block as it would appear 
when seen squarely from that particular side, and it is quite 
evident that, when one view has been drawn, as for example 
the front view, lines may be projected to the other views for 
locating them properly, as will be explained and illustrated by 
practical examples. 

This drawing, at B, Fig. i, is a very simple example of 
orthographic projection. Either style of drawing shown in 
Fig. i might be used to represent such a plain piece as this 
block. For instance, if steel blocks of a certain size were re- 
quired, the necessary dimensions could be placed on the per- 



PROJECTION 



13 



spective drawing and this could be used by a machinist as 
well as a properly dimensioned drawing made according to 
the orthographic projection method, but when a part is of 
irregular shape, and especially if several pieces are combined 
to form some kind of a mechanism, the separate views repre- 
senting the front and, perhaps, the side and top are much 
superior to a perspective drawing. In fact, perspective draw- 
ings would not be at all practicable for most of the work 
represented by mechanical drawings. 




Machinery 



Fig. 2. Diagram illustrating Principle of Orthographic Projection 



Principle of Orthographic Projection Method. — As it is 

essential for the mechanical draftsman to understand 
thoroughly the principle of orthographic projection, this 
method of drawing will be further explained. To begin with, 
the representation of a plain rectangular block by means of 
separate views showing the shape as seen from the front, 
top, and side, will again be considered, because a simple object 
of this kind illustrates the principle better than a drawing of 
some complicated mechanical device. At A, Fig. 2, this 



14 MECHANICAL DRAWING 

block, which is shaded, is represented as being enclosed by a 
box formed of glass sides. Now, if lines were extended or 
projected from the four corners of the block to the front of 
the box, as illustrated by the dotted lines, and these four 
points were joined as shown by the full lines, the square thus 
drawn would correspond to the front view. In the same 
way, if the corners of the side were projected to the side of the 
box and a rectangle drawn, this would correspond to the side 
view of the block. The top view is represented as being 
projected up to the top side of the box in a similar manner. 
These three views now represent a mechanical drawing made 
according to the orthographic projection method, but they 
lie in three different planes and on an actual drawing it is, of 
course, necessary to place all three views on a flat sheet or 
so that they all He in one plane. If it is assumed that the 
top and right-hand side of the glass box are hinged at the 
front edges, and that they are turned so as to lie in the same 
plane as the front side, the views will then appear as shown 
at B or in the same relative positions as the three views illus- 
trated at B, in Fig. i. 

It will be understood that diagram A is intended merely to 
illustrate the principle of orthographic projection and that, 
in actually making a drawing of this block, the front view 
would ordinarily be drawn first to whatever size the block 
happened to be or to some reduced scale; then lines would 
be extended or projected for locating the end lines of the 
side and top views. The rectangles would then be com- 
pleted by drawing lines representing the sides on both the 
top and side views, the distance between these lines corre- 
sponding to the thickness of the block. 

Number and Arrangement of the Views. — A mechanical 
drawing may show only one side of an object or it may be 
composed of two or more views. Two or three views are the 
usual number, although four may be needed and sometimes 
it is necessary to add separate views of important details. 
These detail views are frequently used to show some part 
which is not represented clearly enough in the general views. 



PROJECTION 1 5 

If a single view representing only one side of the part drawn 
is sufficient to show clearly all that is required in making 
this part, additional views would be useless and the time re- 
quired for drawing them would be wasted. 

When there are two or more views, it is evident that if they 
have been simply drawn on a sheet of paper in haphazard 
fashion or without regard to their respective locations, the 
drawing may be very confusing, because it will not be ap- 
parent which view represents the front of the object and 
which ones show the shape and size of the piece as seen from 
the top and end. In other words, the relation between the 
views and the part they represent will not be apparent in all 
cases. For this reason, the views of mechanical drawings 
are arranged according to a definite plan. In the United 
States, the general practice is to place the top view above 
the front view, and the end view next to whatever end it 
represents. For example, if a view of the left-hand end is 
considered preferable to a view of the right-hand end, this 
end view is placed to the left of the front view, thus indicating, 
that it represents the left-hand end or side. If it were con- 
sidered advisable to show both ends, then a right-hand view 
would be placed to the right of the front view. In some in- 
stances, a bottom view is needed, in which case it is placed 
below the front view. 

The view obtained by looking at the object from above is 
known as a plan view; that obtained by looking at the object 
from one of its sides and showing a vertical face is known as 
an elevation, and it may be either a front elevation or an end 
elevation (also known as side elevation) , depending upon whether 
the view is of the front or side of the part drawn. 

In the case of a simple object like a bolt, screw, or washer, 
one view is sometimes sufficient, but in most cases two or 
three views are required, as previously mentioned. The 
number of views depends upon the shape of the object and the 
purpose of the drawing. For example, a screw would be 
shown from the side, and an end view might be drawn to 
show the shape of the head. In the case of a shaft, an end 



i6 



MECHANICAL DRAWING 



view might be included with a side view to show its circular 
form, or the location and size of keyways or attached parts. 
Third-angle and First-angle Projection. — It is generally 
the practice to represent a machine detail in the position that 
it will occupy in the machine for which it is intended, instead 
of showing it upside down or in some other direction. When 
the views are placed with the plan above the front elevation, 
the right-hand end view to the right and the left-hand end 
view (when drawn) to the left, this is known as third-angle pro- 
jection. In European countries, it is frequently the custom to 
use what is known as first-angle projection. With this method, 
the front elevation is placed at the top, the plan view, at the 



LU 
. ... 



THIRD-ANGLE PROJECTION 1 



FIRST-ANGLE PROJECTION 



Machinery 



Fig. 3. Comparison of Third-angle and First-angle Methods of Projection 

bottom, the right-hand end view at the left, and the left- 
hand end view at the right. (The difference between these 
two methods is illustrated in Fig. 3.) The first-angle projec- 
tion is also generally employed in architectural and structural 
work, as in drawings of bridges, etc. 

The Study of Different Types of Drawings. — The mechani- 
cal draftsman must understand the relation between different 
views and what they represent. He must also know what 
views are required to show properly and clearly, by the pro- 
jection method, the form of any mechanical device for which 
a drawing may be required. In order to become proficient 
in the art of making good mechanical drawings and in 
interpreting or reading existing drawings, it is necessary to 
understand the underlying principles and then, by exercising 
judgment, to apply these principles to the different problems 



PROJECTION 



17 



and conditions that may arise. There are no inflexible rules 
that can be laid down and used as a guide either in making 
drawings or as a means of understanding them, but by study- 
ing first very simple drawings and then those that are more 
complex, the various methods of representing practically any 
mechanical device will be apparent The principal point to 
bear in mind when making mechanical drawings is that they 
are to serve as a guide in producing whatever part is drawn. 
For this reason, the draftsman should, as far as possible, con- 
sider the drawing from the viewpoint of the patternmaker and 
machinist who will use it. 




Fig. 4. Simple Example of Projection 

Simple Examples of Orthographic Projection. — Since the 
best way for the student of drafting practice to become fa- 
miliar with mechanical drawing methods is by studying draw- 
ings of various kinds, examples representing distinct types 
will be considered, beginning with the simplest forms. At A, 
Fig. 4, is shown a drawing of a short bar of rectangular shape. 
The view to the left is meaningless until it is combined with 
the end view. The latter shows at a glance that this bar is 
rectangular in cross-section. The sketch B in the same 
illustration represents a half-round bar as is clearly illus- 
trated by the end view. 



i8 



MECHANICAL DRAWING 



Figure 5 represents a drawing of a small cast-iron block 
which has a circular boss projecting from one side. If the 
view to the left were the only one shown, there might be 
doubt as to what the circle represents; although if it were 
shaded, as will be explained later, the fact that the circle 

























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Machinery 



Fig. 5. Another Example illustrating the Relation of Different Views 





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Fig. 6. Illustration showing Use of Dotted Lines for representing Concealed 

Surfaces 



represents a boss and not a hole might be shown quite clearly. 
Even if this much were known, the height of the boss and 
the thickness of the block could not be indicated by this 
single view; but when an end view is added, the proportions 
of the entire casting are clearly revealed. 



PROJECTION 



19 



Before continuing with the examples illustrating different 
types of drawings, the use of dotted lines on drawings as a 
means of representing interior and concealed surfaces should 
be explained, since these lines appear on a great many 
drawings. 

Use of Dotted Lines to Show Interior or Concealed Sur- 
faces. — A great many of the castings, forgings, and other 
parts used in the construction of mechanical devices have 
holes, recesses, and ports or other interior passages of various 
shapes which may be partly or entirely hidden from view, 
especially on a drawing formed only of lines representing 
exterior surfaces. To illustrate how interior surfaces are 

















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Fig. 7. Another Example illustrating the Use of Dotted Lines 

shown, a casting is illustrated in Fig. 6 which is similar to 
the one shown in Fig. 5, except that it has an opening extend- 
ing through the center of the boss. In the view to the left 
(Fig. 6), there are three concentric circles, and by referring to 
the end view, it is easy to determine just what each circle rep- 
resents. The outer circle is the outline of the boss, as in the 
preceding case; the next circle represents the large part of 
the hole, and the smallest circle, the small part of the hole. 
The depth of the large section is also shown clearly by the 
dotted lines. 

The use of dotted lines is further illustrated in Fig. 7, which 
shows a circular pin having five different diameters. By 
referring to the end view, it will be noted that there are three 
circles formed by continuous lines, and two formed by dotted 



20 



MECHANICAL DRAWING 



lines. The relation between these full and dotted circles, 
and the sections shown by the side view to the left, is indi- 
cated by corresponding reference letters. For instance, the 
outer circle B represents the larger collar B x \ the circle D 
represents the collar Di\ the dotted circles A and C repre- 
sent the sections A x and Ci, respectively; and circle E repre- 
sents Ei. The circles A and C are dotted because both the parts 
they represent are concealed or lie back of the collars when 
the pin is seen from the right-hand end. If part A\ were 
larger than B h then circles A and B would both be formed 





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Fig. 8. A Drawing requiring only Two Views 



by continuous or full lines, assuming, of course, that the end 
view were placed on the right-hand side, as in this case. 

Dotted lines representing either interior surfaces or parts 
which are back of some other section are found on almost all 
mechanical drawings. In a great many cases, however, the 
shape of an interior opening or of a concealed surface is shown 
by one or more sectional views instead of using dotted lines. 
The use of sections is explained in Chapter V. 

Drawings Requiring Only Two Views. — A great many 
drawings of simple parts require only two views. Some 
drawings of this kind have already been referred to. The 



PROJECTION 2 1 

small rectangular block shown in Figs. 5 and 6, which has a 
circular boss projecting from one side, is represented very 
clearly by two views. If a plan view were added to the draw- 
ing, it would not show anything other than what is repre- 
sented by the two views given. In this case, however, a plan 
or top view might be substituted for the end view. If the 
sides of the block were tapering instead of being parallel, 
then a plan view would be needed to show the tapering sides, 
but it would not be necessary to show an end view. From 
this it will be inferred that when there are only two views, 
the two sides are shown which represent the part to the best 
advantage. In some cases, front and plan views are needed, 
and in others, front and end or side views. 

Another drawing requiring only two views is shown in 
Fig. 8. This drawing is of a cast-iron knee and it contains 
all the dimensions of the knee. The view to the left shows 
that the vertical part of the knee is square with the base, and 
it shows the height and width of the vertical section as well 
as the length and thickness of the base. The dotted line at 
A also indicates that there is an opening through the vertical 
part, but without an end view, it would not be possible to 
tell anything about the shape of this opening. By referring 
to the end view, it is evident that the opening or slot is rec- 
tangular in shape and that it is if inches wide and 2 inches 
deep. This end view also shows that the base of the knee 
has a projecting section or "tongue" f inch wide and | inch 
deep. With a drawing of this kind, a patternmaker could 
easily make the pattern needed for producing the casting, 
and then this same drawing could be used by the machinist 
when planning those surfaces which must be finished accu- 
rately to the given dimensions. 

Drawings Requiring Three Views. — Mechanical drawings 
composed of three separate views of whatever part is shown 
are very common, especially in the case of shop drawings of 
machine details, tools, etc. Sometimes a third view is added 
merely to make the drawing clearer and more easily under- 
stood than one having only two views, but in many instances 



22 



MECHANICAL DRAWING 



three views are absolutely necessary in order to show every 
part of the object in its true form. An example illustrating 
why three views are sometimes required is shown in Fig. 9. 
This drawing represents a cast-iron knee which is the same 
as the one previously referred to, except that it has an oblong 
slot through the base which is surrounded by a boss of simi- 
lar shape. In this case, if the top or plan view were omitted, 





























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Fig. 9. A Drawing requiring Three Views 

the two remaining views would show the length and width of 
the boss and of the opening through the base, but it would 
not be possible to determine whether or not the boss and 
opening were rounded at the ends or square, although 
it would, of course, be reasonable to assume that they 
were rounded. By adding a plan view, one can see at a 
glance the exact shape of the opening and the boss which 
surrounds it. 



PROJECTION 



23 



Another piece which requires three views to represent it 
fully is shown in Fig. 10, which is a drawing of a drop-forging. 
The dotted lines in the front view show that there is an open- 
ing through the forging, and the plan view shows that this is 
a slot having rounded ends. If there were only a plan view, 
it would not be possible to determine the shape of sections A, 
B, and C, but by referring to the end view, it is clear that 
the part A is square and parts B and C circular in cross-section. 
As the circle D is solid and not dotted, this indicates that it 
represents the end C and not the part A. On the contrary, 
if part A were circular and C square, then part A in the end 




Fig. 10. A Drop-forging represented by Three Views 



view would be concealed by part C and the circle D repre- 
senting it would be dotted. This end view illustrates the 
importance of dotted lines, and shows how even a slight change 
of the kind mentioned may modify the shape of a part as 
represented by the drawing. 

The cast-iron intake manifold shown in Fig. n is part of a 
gasoline engine, and is still another example illustrating a 
drawing requiring three views. The need of three views in 
this case is quite apparent. The front view shows the curva- 
ture of the pipe in a vertical plane, but its curvature in a 
horizontal plane is not shown at all by this view. The plan 



24 



MECHANICAL DRAWING 



view illustrates how the main branch curves at each end, 
but it does not show the complete shape, and it is necessary 
to add an end view. By referring to these three views, it is 
clear that the manifold has round flanges at A and a round- 
cornered diamond-shaped flange B. The shape of flange B 
is shown in the plan view where dotted lines are used to repre- 
sent that part which is concealed beneath the main branch 
of the manifold. The bosses or lugs C, D, and E should also 
be referred to in the different views, as they illustrate how such 
details are represented. As this manifold is a hollow casting, 
the passageways are shown by dotted lines. 




Fig. 11. Drawing of an Exhaust Manifold 

Auxiliary Views of Inclined Surfaces. — According to the 
method of projection illustrated by the diagram A } Fig. 2, 
the three planes upon which the front, side, and top views are 
projected are at right angles to each other and each plane is 
parallel to that surface of the block the outline of which is 
projected upon it; moreover, each view represents the true 
size of that particular side, because every line or edge on the 
block is parallel to the plane upon which it is projected. 

In drafting practice, it is often necessary to make a draw- 
ing of some casting or forging which has inclined surfaces or 



PROJECTION 



25 



parts that would not be shown in their true proportions by 
views of the kind previously referred to; hence, it is neces- 
sary, in many cases, to draw auxiliary views which show the 
inclined part just as though it were projected upon a plane 
parallel to it. One or two examples will serve to illustrate the 
idea more clearly. 

Figure 12 shows the drawing of a small casting that has an 
inclined or beveled side in which there is a recess and hole. 




Fig. 12. Illustrating Use of Auxiliary View of Inclined Surfaces 

The plan view does not show this inclined part clearly nor 
in its true proportions, because the inclined edges, as seen 
in the plan view, are foreshortened or less than their actual 
length; but by drawing an auxiliary view, as shown, which 
represents the inclined part of the casting as it would appear 
when seen squarely and not at an angle, the true form is clearly 
represented. This auxiliary drawing is simply a plan or top 
view of the inclined part of the casting, or, in other words, 
it corresponds to the view that would be obtained if the 
lines were projected upon a plane parallel to the beveled 
surface. 



26 



MECHANICAL DRAWING 



The bellcrank shown in Fig. 13 also has an auxiliary view. 
The two arms of the bellcrank are at an angle of 60 degrees, 
and they are offset. Now if both arms were alike, the aux- 
iliary view would not be needed. It would simply be neces- 
sary to give the dimensions for one arm, as shown in the 




Machinery 



Fig. 13. Drawing of a Bellcrank 



left-hand view, and then place a note on the drawing explaining 
that the other arm is a duplicate. In this case, however, a 
special detail drawing is needed. As will be seen, it shows 
the inclined arm just as if it were viewed squarely from the 
side; consequently, the length of the arm and the curves are 
represented in their true length and form. 



CHAPTER III 
MECHANICAL DRAWING INSTRUMENTS AND MATERIALS 

Mechanical drawings should be fairly accurate, particu- 
larly when the drawing is relied upon to show the relative 
location of certain parts of a mechanism. While the drawing 
itself should have all important dimensions marked on it so 
that it need not be measured to obtain sizes, nevertheless a 
drawing which is accurately proportioned is usually desirable 
and often necessary. Any error in the calculations employed 
to determine the dimensions of members composing a mecha- 
nism may be detected by a correctly proportioned drawing, 
and accuracy is especially desirable for drawings of compli- 
cated mechanisms. 

To obtain the required degree of accuracy, it is necessary 
to use mechanical drawing ' instruments which include types 
for drawing straight lines, circles, and lines at given angles in 
accordance with required measurements. This chapter does 
not deal with the different special constructional features of 
various grades of drawing instruments or with the use of 
special tools employed only rarely by draftsmen, but it is a 
general review of the types ordinarily required in making 
mechanical drawings and of the methods of using the various 
instruments. 

Pencil and Ink Drawings. — It is common practice to 
make pencil drawings on paper first and then copy the pencil 
drawings in ink on some transparent material, such as tracing 
paper or tracing cloth placed over the pencil drawing. The 
inked drawings, commonly called " tracings," are then used 
to make any required number of prints or reproductions for 
use in the shop. In order to have the lines show clearly and 
distinctly on the print, it is necessary that the inked lines on 
the tracing be fairly heavy. While the making of tracings 

27 



28 



MECHANICAL DRAWING 



and prints is the most common method, the pencil drawing is 
sometimes inked in directly on the paper. This method, for 
instance, is often used in making patent office drawings. In 
either case, the making of pencil and ink drawings requires 
the use of instruments especially designed for drawing straight 
lines and circles, and means of securing the proper proportions 
between the different parts represented. 

The Ruling Pen. — Most drawings are composed largely of 
straight lines which are drawn by using the straight edge of 
a T-square or a triangle as a guide for the pen or pencil. In 




Fig. 1. Rxiling Pen for drawing Straight Lines 

the case of ink drawings, the instrument used is known as 
the " ruling pen." The ruling pen, Fig. i, has two steel blades 
having points or "nibs" which are rounded as shown at a. 
The nibs or points should have fairly sharp edges, and should 
be of equal length and of the same form. The space b between 
the points contains the ink (which is represented by the solid 
black portion) and the width of this space determines the 
width of the line drawn; the space can be varied for drawing 
fine lines or heavy lines of uniform widths, by means of the 
adjusting screw c. The pen should be held so that both 
points rest on the paper, and it is necessary that they be 



INSTRUMENTS AND MATERIALS 



29 



equally sharp in order to produce fine lines. The T-square 
or triangle which is used to guide the ruling pen in drawing 
straight lines is represented by d. It will be observed that 
the ruling pen is guided only by the top edge or corner of d 
and that there is a space e between the point of the pen and 
the lower corner of d. This space is necessary in order to 
prevent the ink from causing blots by coming into contact 
with the guide d. It is evident that the distance e must be 
kept uniform during the entire stroke of the pen in order to 




Fig. 2. Method of using Ruling Pen 



produce a straight line and also to prevent either of the points 
of the pen from leaving the paper. 

The ruling pen is filled with ink by inserting the quill of the 
ink bottle stopper between the pen points. Care must be 
taken not to put too much ink into the pen, and it is prefer- 
able to place the ink bottle so that the pen can be filled with- 
out holding it over the drawing-board. When the pen is 
filled in this manner, no ink should be found on the outside 
of the pen blades; however, if any ink should accumulate 
on the outside of the blades, it should be removed with a 
piece of linen cloth before attempting to use the pen. Ruling 



3° 



MECHANICAL DRAWING 



pens should be carefully cleaned after using as ink, if allowed 
to dry on them, will cause corrosion. The inside of the blades 
should be wiped frequently before refilling. 

Drawing Straight Lines. — In Fig. 2 is shown a front and 
a side view of a ruling pen held by the hand in the correct 
position for drawing straight lines. Only a little practice is 
required in order to enable anyone to draw straight . lines 
neatly and rapidly. Horizontal straight lines should always 
be drawn from left to right, care being taken to hold the rul- 




Fig. 3. (A) Bow Pencil. (B) Bow Pen- 



ing pen in the straight upright position during the entire 
stroke. With the exception of very short lines, a full arm 
movement should be used. When within about one half 
inch from the point at which a line is required to terminate, 
the full arm movement should be discontinued and, with the 
tips of the fingers resting upon the top of the straightedge, 
the pen should be brought to a stop at the desired point simply 
by the motion of the fingers which hold it. 

During the entire stroke in which the arm movement is used, 
the fingers should touch the top of the straightedge lightly in 



INSTRUMENTS AND MATERIALS 



3 1 



order to steady the hand. The pen should not be gripped 
too tightly and should be held in contact with the straight- 
edge with only sufficient force to prevent it from leaving the 
guiding edge. The pen is usually inclined in the direction in 
which it is moving as indicated in Fig. 2. Some draftsmen, 
however, prefer to hold the pen in a perpendicular position. 
Short lines should be drawn with the finger motion only, 
such as used at the end of the full arm movement. 

Instruments for Drawing Circles. — The instrument used 
for drawing circles depends somewhat upon the size of circle 




Fig. 4. Setting Bow Pencil to a Given Radius 



to be drawn. The spring bow pencil and spring bow pen are 
ordinarily used to draw circles up to 1 \ or 2 inches in diam- 
eter. The construction of spring bow instruments varies 
slightly, but they are all alike in their more important details. 
They consist essentially of two steel legs a and b, as shown 
in Fig. 3, which are so constructed that a spring tension forces 
their lower ends apart against the adjusting nut c. Handle d 
provides a convenient means of holding the instrument when 



32 MECHANICAL DRAWING 

adjusting it or when drawing circles. The leg b of the spring 
bow pencil shown at A is fitted with a steel bar which ter- 
minates in a fine steel point, and the leg a is so constructed 
that a piece of lead g can be secured in the position shown. 
The lead and the needle point should be so adjusted that the 
handle d is perpendicular to the paper or drawing-board when 
the needle point is set in the paper and the pencil point rests 
on the paper in the position shown. The needle point has a 
shoulder which allows the point to pierce the paper only a 
short distance. 

With the exception of the leg a, the spring bow pen shown 
at B is the same as the bow pencil. The leg a is fitted with 
a pen for drawing circles in ink. The pen is similar to the 
ruling pen and the distance between the points or "nibs" 
can be adjusted by the screw e. 

Drawing Circles with Spring Bow Instruments. — When 
drawing circles to a given diameter, the bow pencil is usually 
set to the radius of the circle by placing it on a suitable scale. 
The instrument should be held in the position shown in Fig. 
4, and the thumb and middle finger should be used to turn 
the nut which adjusts the distance between the needle point 
and the pencil point. When drawing a circle, the hand should 
be raised until the thumb and index-finger assume the posi- 
tion shown in Fig. 5. The circle is then drawn by rolling the 
handle of the instrument between the thumb and index-finger 
in a clockwise direction. The handle should be inclined 
slightly in the direction of rotation. The center of the circle 
is ordinarily located at the intersection of two center lines. 
If the center is at some other point, a small circle drawn free- 
hand with a lead pencil about the center prick mark will 
enable its position to be located quickly when making the 
tracing. The bow pen is usually set by adjusting it with 
reference to the circle drawn in pencil. 

Some draftsmen make a practice of changing the setting of 
spring bow instruments by pressing the points of the instru- 
ments together with the thumb and index-finger of the left 
hand, and spinning the adjusting nut with a finger of the 



INSTRUMENTS AND MATERIALS 



33 



right hand. In this way, the instrument can be quickly 
closed or opened to the maximum capacity. This method is 
used only to obtain approximate adjustment and the final 
setting should always be made with one hand alone, as pre- 
viously described. 

The Compass. — The compass is used for drawing circles 
that are too large to be made with the bow pencil or bow pen. 
At A, Fig. 6, is shown a compass with pen in place and the 
needle point properly adjusted. The instructions on the 




Fig. 5. Drawing a Circle with a Bow Pen 

care of the ruling pen also apply to the pen used in the com- 
pass. The needle point should be adjusted so that the point 
of the pen is even with the shoulder on the needle-bar as 
shown. The needle-bar should be locked in this position 
where it should remain permanently. The pen point can be 
readily removed and replaced by the pencil point shown at C, 
when pencil lines are required. The lead should be adjusted 
to agree with the setting of the needle point. When circles 
larger than about 3 inches are to be drawn, the points of the 
compass should be adjusted at the joints, as shown at D, in 



34 



MECHANICAL DRAWING 



order to bring these members perpendicular to the paper or 
drawing-board. The extension bar shown at B is used in 
drawing circles which are too large to be drawn with the 
compass when equipped only with the pencil point or pen 
point. This lengthening bar is inserted between the leg of 
the compass and the pen or pencil point. 

Drawing Circles with the Compass. — As the compass is 
frequently used, the correct methods of handling this im- 




Fig. 6. Compasses for drawing Circles which are too Large for the Spring 
Bow Instruments 

portant instrument should be acquired. When the compass 
is picked up, the thumb should be placed in the position 
shown in Fig. 7. This position permits the instrument to be 
readily opened by pressing with the thumb and middle ringer 
into the chamfered section. When the compass is opened a 
sufficient amount, the ringers of the right hand will naturally 
assume the position shown in Fig. 8. When it is held in this 
position, the opening of the compass can be readily continued 
for setting the points to coincide with graduations on a scale. 
This enables the draftsman to make all adjustments required 



INSTRUMENTS AND MATERIALS 



35 




Fig. 7. Position of Hand for opening Com- 
pass readily when it is picked up from 
Drawing-board 



in drawing circles up to 
about 3 inches radius, with 
the right hand alone. For 
larger circles, the compass 
should be opened and 
closed by the same method, 
but the thumb and index- 
finger of the left hand 
should be used to bring 
the jointed sections into 
such a position that they 
will both be perpendicular 
to the paper as shown at 
D, Fig. 6. The final ad- 
justment is then made by 
using the fingers and thumb 

of the right hand alone, the instrument being held as previously 
described in connection with Fig. 8. When drawing a circle, 
the position of the hand is changed so that the top or handle 
of the instrument will be held between the thumb and index- 
finger, the same as the bow instrument is held in Fig. 5. 
The circle should be drawn by rotating the instrument in a 

clockwise direction. In 
drawing heavy ink circles, 
it is usually necessary to 
begin the rotative move- 
ment before allowing the 
pen point to touch the 
paper, as otherwise the ink 
may leave the pen and 
cause a blot. In Fig. 9 is 
shown the method of us- 
ing the compass equipped 
with a lengthening bar for 
drawing circles that are 

Fig. 8. Setting Compass toGiven Radius t0 ° lar S e t0 be draWn **& 

a compass alone. When 




36 MECHANICAL DRAWING 

the lengthening bar is employed, the instrument should be 
held by both hands. 

Beam Compass. — Circles beyond the range of an ordinary 
compass are drawn with the beam compass shown at E, Fig. 6. 
This is the usual type of beam compass and consists of a beam 
or strip of hard wood carrying two heads provided with needle 
points, a pencil-holder, and pen. When a pencil or pen is 
being used, the head carrying the needle point is usually 
clamped at one end of the beam or bar while the one carry- 




Fig. 9. Drawing an Arc of rather Large Radius by using Compass and 
Lengthening or Extension Bar 

ing the pencil-holder or pen is adjusted at any point along 
the beam that may be required for the radius of the arc or 
circle to be drawn. Only a small section of the beam is shown 
in the illustration. 

Dividers. — Dividers are used for dividing circles or straight 
lines into equal parts and for transferring measurements from 
a scale to the drawing. For making small and very fine divi- 
sions, the spring bow dividers shown at A, Fig. 10, are used. 
The construction is similar to that of the bow pen and bow 
pencil, the only difference being that both legs terminate in 
sharp needle points. The method of holding and adjusting 



INSTRUMENTS AND MATERIALS 



37 



the spring bow dividers is essentially the same as that for 
the bow pen and bow pencil. The fine adjustment provided 
on the spring bow dividers makes them particularly adapted 
for making divisions requiring a fair degree of accuracy, such 
as is often required in dividing a circle into any number of 
equal parts. 

Dividing a Circle into a Given Number of Equal Parts. — 
When the distance between the points is set as nearly as pos- 




Fig. 10. Different Kinds of Dividers 



sible to the length of one of the required spaces, by either 
estimate or scale measurement, the line or circle should be 
spaced off by a trial division. The spacing should be done 
in the usual way, except that the points of the instrument 
should not be pressed into the paper. When spacing or divid- 
ing lines or circles, the dividers should be held between the 
thumb and forefinger, each point of the dividers being moved 



38 MECHANICAL DRAWING 

alternately along the line to be divided. If the trial division 
proves that the instrument is properly set to make the re- 
quired number of equal spaces, the spacing can be done by 
pressing down on the instrument with just sufficient force 
to make easily distinguished prick marks. If the spacing 
does not come out even, open or close the instrument a dis- 
tance equal to the amount of error divided by the number of 
divisions and make another trial division. Repeat this opera- 
tion until the instrument is so adjusted that the line can be 
divided exactly as required. 

Plain, Hairspring and Combination Dividers. — The plain 
dividers shown at B, Fig. 10, are used for making divisions 
beyond the capacity of the spring bow dividers. This instru- 
ment consists of two steel members joined at the top by a 
friction joint similar to that of the ordinary compass. The 
lower ends of the dividers have very sharp steel points for 
making small prick marks. The dividers should be opened, 
closed, and adjusted by the fingers and thumb of the right hand 
alone just as in the case of the compass. 

The type of dividers shown at C, Fig. 10, are known as hair- 
spring dividers. In this type of dividers one leg is made solid 
while the other is hinged or jointed and is provided with an 
adjusting screw that permits of very fine adjustment. 

The dividers shown at D are formed by replacing the needle 
point and pen or pencil point of the ordinary compass with 
solid steel points. These divider points are not often used 
as most draftsmen have either plain or hairspring instruments. 

Proportional Dividers. — Proportional dividers are useful 
when enlarging or reducing drawings by direct measurement, 
and for dividing a circle into equal parts. They are made 
with two entirely separate legs, which have steel points at 
each end and are joined to each other by a screw and thumb- 
nut sliding in a slot formed in each leg, as shown in Fig. 11. 
The pivot screw passes through a sliding block formed of 
two parts, each fitting the slot in its respective leg, so that 
the joint, or pivot, of the instrument can be placed at any 
point desired. Thus the double-pointed legs form practically 



INSTRUMENTS AND MATERIALS 39 

two dividers, the relative lengths of the legs of which are ad- 
justable at will. If the pivot screw is so placed that its dis- 
tance from one point is one third of the entire length of the leg 
from point to point and is clamped in that position, the divid- 
ers are set at a proportion of i to 2; that is, if the divider 
legs are opened until the points of the shorter leg are one inch 
apart, the points of the longer will be two inches apart. By 
shifting the position of the pivot screw, any other relative 
proportion can be obtained. The position of the pivot screw 
is determined by graduations upon the legs, to which a single 
line upon the sliding block may be adjusted. When it is 
desired to enlarge or reduce a drawing, the scale of gradua- 
tions marked "lines" is used; the scale marked " circles " is 
used when it is desired to divide a circle into a number of 




Fig. 11. Proportional Dividers 

equal parts, the diameter of the circle being measured by the 
large end of the dividers. In mechanical drafting, this instru- 
ment is not ordinarily used. 

The T-square. — Since most drawings consist principally 
of horizontal and vertical lines of various lengths, a con- 
venient means of drawing them is necessary. Horizontal 
lines are usually drawn by guiding the pen or pencil with the 
edge of a T-square blade. The T-square consists primarily 
of a thin ruler or blade which has a head secured to it at one 
end. The head is usually fixed at right angles to the ruling or 
working edge of the blade. This type is shown at A, Fig. 13. 
The head is held against the edge of the drawing-board with 
the left hand. The T-square shown at B has a swivel or 
pivoted head that may be secured in any desired position by 
a thumb-nut. This type of T-square is sometimes equipped 
with a protractor for setting the head at any required angle. 

3L 



4° 



MECHANICAL DRAWING 



In Fig. 12, the T-square is shown in its normal position on 
the drawing-board. Ordinarily all horizontal lines are drawn 
by the aid of the T-square which is moved as may be necessary, 
the head always being held in contact with the left-hand edge 
of the drawing-board. 

Triangles. — Vertical lines are usually drawn with the aid 
of triangles which are generally used in connection with the 
T-square. There are two types of triangles that are com- 
monly used by mechanical draftsmen. These two forms are 




Fig. 12. Drawing a Straight Line by using T-square to guide Pencil 

shown at A and B, Fig. 14. The one shown at A is commonly 
called the 45-degree triangle and has one angle of 90 degrees 
and two angles of 45 degrees. The triangle shown at B is 
usually called the 60-degree triangle and has one angle of 90 
degrees, one of 30 degrees, and one of 60 degrees. Triangles 
are usually made from | to f inch in thickness and from 3 to 
15 inches in length, according to the size of drawing for which 
they are to be used. They are made of celluloid and of vari- 
ous kinds of hard wood, hard rubber, or similar substances. 
The celluloid ones are transparent and are preferable. 
Straight lines may be drawn at right angles to the hori- 



INSTRUMENTS AND MATERIALS 



41 




Machinery 



Fig. 13. T-squares of Fixed-head and Adjustable Types 

zontal edge of the T-square or at angles of 45 degrees, 30 de- 
grees and 60 degrees by placing the triangles in the positions 
indicated by the dotted lines in Fig. 14. In Fig. 15 is shown 
the correct position of- the hands in drawing vertical lines. 
This position permits the triangle to be held in contact with 
the T-square and it also enables the draftsman to keep the 
head of the T-square in contact with the edge of the drawing- 
board. The 4 5 -degree triangle is almost universally employed 
for cross-sectioning or cross-hatching sectional drawings. 




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Fig. 14. Triangles in Different Positions on T-square 



42 MECHANICAL DRAWING 

There are various special instruments designed for cross- 
sectioning, but experienced draftsmen usually prefer to use 
the plain 45-degree triangle for ordinary work. For special 
work, triangles having different angles are used, but nearly 
all of them have one angle of 90 degrees. The 60-degree tri- 
angle is used principally for drawing hexagons, such as are 
required in representing bolt heads, etc. 

The size of a triangle is determined by the length of the 
side of the right angle, the longer side being measured in the 



Fig. 15. Drawing a Vertical Line by using T-square and Triangle 

case of the 30- by 60-degree triangle. The different angles 
which can be laid off by using the 45- and 30- by 60-degree 
triangles singly and in combination are shown by the illus- 
tration, Fig. 16, which is self-explanatory. 

Protractors. — The protractor is used to measure or set off 
angles of any required number of degrees. The protractor 
shown in Fig. 17 is a type often used by draftsmen. It is 
made from celluloid and includes one half of a circle. Some 
protractors are provided with an arm pivoted at the center 
which can be swung around the circle (see Fig. 18). The 



INSTRUMENTS AND MATERIALS 



43 



smaller protractors are divided into degrees, while larger ones 
show half or quarter degrees. Those with a swinging arm 
like the one shown in Fig. 18 are usually provided with a 
vernier scale by which small fractions of degrees, such, for 
example, as i, 3, or 5 minutes, are read. 

Irregular Curves. — Irregular curves, sometimes called 
" French curves," are required in drawing curved lines other 




Fig. 16. Different Angles which may be laid off by using the 45- and 
30- by 60-degree Triangles 



than circles or arcs. Curves are generally made of celluloid 
and in a great variety of sizes and forms. The curves may 
be parts of ellipses, spirals, parabolas, or other curves. For 
special purposes, they are made of any curve called for by 
the drawing to be made. Typical curves are shown in Fig. 19. 



44 



MECHANICAL DRAWING 




Fig. 17. Simple Form of Protractor 

One elliptical and one spiral curve will serve for general pur- 
poses. In practice, the points through which a. curved line 
is to be drawn are located and then the edge of the irregular 
curve is made to coincide with as many of these points as 
possible and a smooth line drawn; then the curve is applied 
to as many more of the points as possible and the curved line 
continued, this process being repeated until the entire irregu- 
lar curve is drawn. 




Fig. 18. Protractor having a Pivoted Arm and Vernier Scale 



INSTRUMENTS AND MATERIALS 



45 



Many curved lines drawn by means of the irregular curve 
require that the same parts of the instrument be used on each 
side of a center line. The regularity of the curve drawn and 
the degree of symmetry will depend upon the accuracy with 
which the irregular curve is located in corresponding positions 
on opposite sides of the center line. One method of locating 
the irregular curve is by making pencil marks on the curve to 
indicate the section used. The instrument can then be re- 
versed and this section used to draw the corresponding curve 
on the opposite side of the center line. A convenient method 




Fig. 19. Irregular Curves used for drawing Curved Lines other than Circles or Arcs 

of drawing a symmetrical figure which requires a right-hand 
and a left-hand curve on each side of a center line, is illus- 
trated in Fig. 20. 

As can be seen, there is a hole about j$ inch in diameter in 
each end of the curve. In use, the curve is laid on the draw- 
ing, the locations of the holes are marked with a pencil point, 
and the desired curve drawn on one side. On the center line of 
the piece to be drawn, select two centers, as A and B, and from 
these centers locate the positions of the holes in the opposite 
sides. Place the curve in the reversed position with the holes 
in the curve over these points. The method is simple; in 
fact, it takes a much longer time to explain it than to follow it. 



4 6 



MECHANICAL DRAWING 



There is only one condition under which the end of a curve 
can be joined to another curve (or to a straight line) so that 
the two lines will join neatly together, and that is where both 
the lines are tangent to the same radius where they join. In 
any other case, there will be a break or sharp place which 
will be apparent to the eye, and further, a piece made after 
the drawing will not be so strong as though the curve joined 
evenly. There is a simple way to obtain this desired end 
and that is to draw at various points on the irregular curves, 
radial lines (or in the case of concave curves, prolongations of 




Fig. 20. Method of drawing a Symmetrical Figure requiring Right-hand 
and Left-hand Curves on Each Side of a Center Line 

radial lines) which are at right angles to the tangents of the 
curves at the points chosen. Figure 21 shows an irregular 
curve on which radial lines perpendicular to the tangents are 
drawn. The curves drawn at the lower part of the illustra- 
tion indicate how neatly curves may be joined when the 
radial lines are used as guides. 

Celluloid Templets. — Where the same work must be fre- 
quently drawn, templets made from some light, easily cut 
material not only facilitate the drawing but make the work 
uniform. While paper or cardboard may be used for tern- 



INSTRUMENTS AND MATERIALS 



47 



porary work, the most satisfactory templets are cut from 
celluloid about o.oi inch thick. A dark tint is preferable to 
white, because of its greater contrast with the drawing paper 
or tracing cloth. The templet shown at A, Fig. 22, is for 
drawing bolt heads and nuts; lines must be ruled on this to 
assist in the correct placing on the center line of the part in 
the drawing. The templet for machine handles of the more 
common size (see sketch B) will also be found useful; in this 




Fig. 21. Method of drawing Curves which are Tangent to the Same Radius where 

they join together 

case, only half of the design is cut out of the edge of the tem- 
plet; the other half of the drawing is made by placing the 
templet on the opposite side of the center line. 

Scales Used by Draftsmen. — Ordinarily the draftsman's 
scale is either flat with beveled edges or triangular with the 
flat sides relieved by semicircular grooves. The flat form of 
scale shown at A, Fig. 23, has both edges beveled to an acute 
angle so that the graduations and figures are easily read. 
While the flat scale has two faces upon which graduations can 
be made, the triangular scale shown at B has six faces for 
graduations. The type shown at B, however, has certain 



48 



MECHANICAL DRAWING 



disadvantages one of which is the difficulty often experienced 
in locating any particular scale. The type shown at A is 
preferred by most draftsmen as it lies flat upon the drawing- 
board and permits either scale to be located quickly. 

There are two general classes of graduations. The first 
consists of graduations for "full size" drawings, which are 
drawings made the same size as the actual parts they repre- 
sent; the second covers graduations that are adapted for 
drawings made on a scale much smaller than the parts repre- 
sented. In the first class, the inches may be divided into 




Fig. 22. Celluloid Templets for drawing Bolt-heads and Machine Handles 

eighths, sixteenths, thirty-seconds, and sixty-fourths, or in 
tenths and hundredths; in the second class, the main gradua- 
tions represent feet, and one foot on these reduced scales may 
actually measure i| inch, 3 inches, or some other fractional 
part of a foot. The use of scales will be explained fully in a 
following chapter. 

Set of Drafting Tools. — The selection of the proper tools 
or instruments is very important. Few draftsmen agree fully 
as to what constitutes a complete set of tools, or about the 
best construction of the various appliances. It is also evi- 
dent that the draftsman must be guided somewhat by the 



INSTRUMENTS AND MATERIALS 49 

class of work he is doing. Certain tools which may be re- 
quired by the work carried out by one designer may not be 
of any use to another. In general, however, the requirements 
are fairly similar, and in the following is given a specification 
of a complete set of tools purchased by an experienced drafts- 
man, for his own use. Undoubtedly his judgment and experi- 
ence may give some valuable suggestions as to the selection 
of the tools needed by any draftsman. 

In the set to which reference is made, the three bow instru- 
ments are 3! inches long with center adjustment. The bow 



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Fig. 23. Scales used by Draftsmen 

pen will draw perfect circles from less than -^ inch diameter 
to 3! inches diameter. There is one 3^-inch pencil compass 
with fixed pencil and needle points, and a 5^-inch pen com- 
pass with fixed pen and needle points, and hairspring adjust- 
ment. The pencil compass is small, because it is preferable 
to use trams for all pencil work beyond its limits, but the pen 
can be used to advantage in the 5j-inch size. The hairspring 
adjustment on it is a great convenience, although by no means 
necessary. The two ruling pens are 5 inches and si inches 
long. The trams consist of a tubular German silver bar in 
three sections, held together by long slip joints, and will work 
to a radius of 50 inches. Both heads slide on the bar, being 
clamped in the desired position with thumb-screws, and the 
points are adjustable in either head. The delicate adjust- 



50 MECHANICAL DRAWING 

ment is of the swinging lever type, which is the only satis- 
factory one for trams. There are two divider points, and 
pen, pencil, and needle points. The whole instrument is very 
stiff and light and has a bar long enough for all ordinary work. 

Most draftsmen prefer the transparent triangles. A good 
selection consists of a 1 6-inch, 30 and 60 degrees, a 10-inch, 
30 and 60 degrees, and a 5-inch, 45 and 45 degrees, for all 
ordinary work. A set of flat scales is far superior to the ordi- 
nary triangular scale. This particular set comprises 7 scales, 
i) I? i j i 2j 3) 6, and 12 inches = 1 foot. These scales are of 
the reverse bevel type, and both sides of each scale are gradu- 
ated the same, but read from opposite ends. With this ar- 
rangement, it is never necessary to do more than turn the 
scale over to have it reading in the desired direction. The 
divided foot on the ij- and 3 -inch scales is marked 2-4-6 
etc., instead of the usual 3-6-9, which makes it easier to find 
the desired point. The 6-inch scale is fully divided into six- 
teenths and the 12-inch, into thirty-seconds. Scales of this 
kind, however, are made only to order by the firms manu- 
facturing draftsmen's scales. A slide rule, a protractor, and 
a couple of curves complete the set of tools. 

Drafting Machine. — The term " drafting machine' ' has 
been applied to a special instrument which is designed to take 
the place of the T-square, triangles, scale, and protractor, in 
order to facilitate the work of drafting. The drafting ma- 
chine illustrated in Figs. 24 and 25 consists primarily of two 
parallelograms, a protractor, and a square having graduated 
ruling edges. The two parallelograms joined together con- 
stitute an arm which, when anchored to the drawing-board as 
shown, gives the protractor and square a parallel motion 
about the drawing. This form of parallel motion permits 
either zero point on the ruling edges to be placed instantly 
at any point on the drawing by a single direct movement, 
due to the fact that the arm formed by the parallelograms is 
similar to the human arm. Thus the action is the same as 
when the hand is moved to any position. 

Starting from zero, a line can be drawn along the graduated 



INSTRUMENTS AND MATERIALS 



51 




Fig. 24. Universal Drafting Machine 




Fig. 25. Drafting Machine counterpoised with Springs for Use on Board placed 
in Vertical Position 



52 



MECHANICAL DRAWING 



ruling edge to just the exact length required. Thus in draw- 
ing a straight line to any required length, there is no chang- 
ing from a ruling edge to a scale edge and there is no over- 
running end to be erased or redrawn. The zero on the scale 
is simply moved to position by a single direct movement of 
the hand which controls the instrument, while the other hand 
is left free to draw the line its exact length. The square is 
used for the reason that as soon as a line is drawn, another 
line at right angles to it is usually required. Considerable 







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Fig. 26. Protractor of Drafting Machine 

time and attention is thus saved, particularly in angular 
work. The blades of the square are interchangeable for all 
graduations. Straightedges can be inserted in place of the 
scales when inking in drawings or tracings. 

A conveniently arranged protractor permits the square 
formed by the two scales to be set at any given angle. The 
square, when set at an angle as shown in Fig. 25, has the 
same parallel motion about the board as when set at zero. 
This feature is of great advantage when making drawings of 



INSTRUMENTS AND MATERIALS 53 

angular work. No matter what the angle may be, it is a 
simple matter to set the protractor, move the zero on the 
scale to position, and draw the line the exact length. In 
Fig. 26 is shown the protractor. The centrally located wood 
handle is of such a shape that it fits into the hand nicely. 
An automatic stop makes it unnecessary to read or clamp the 
protractor at the most frequently used angles. For instance 
if it is required to set the square at an angle of 30-degrees 
with the horizontal, it is only necessary to press the thumb- 
latch shown at the right of the wood handle and rotate the 
handle a sufficient amount to allow the latch to drop into a 
recess which automatically and accurately locates the square 
at the required angle. From this it will be seen that the 
protractor is the controlling center of the instrument. The 
handle is held in the left hand which controls all of the mo- 
tions of the machine, thus leaving the right hand free for 
drawing lines. 

As the draftsman becomes accustomed to placing the zero 
on the scale in position by a single direct movement of the 
left hand and drawing the line just its exact length with the 
right hand, he steadily gains in concentration and speed, 
which results in a high degree of efficiency. Protractors of 
special design can be obtained for architectural work or map 
drawing and can be equipped with a vernier which reads to 
minutes. The type shown in Fig. 24 is the one ordinarily 
used in the drafting-room for making drawings of the ordinary 
size when the board is placed on a table or stand in the usual 
horizontal or inclined position. The instrument shown in 
Fig. 25 is equipped with a counterpoise and is used when the 
board is held in a vertical position. For very large drawings, 
the counterpoised instruments are equipped with special 
holders that enable them to be moved to any position on the 
board without affecting their accuracy of alignment. The 
drafting machine illustrated is made by the Universal Draft- 
ing Machine Co., Cleveland, Ohio. 

Drawing-boards. — The kind and variety of the equipment 
used by mechanical draftsmen vary somewhat according to 



54 MECHANICAL DRAWING 

the number of drawings made, their size, and the general con- 
ditions under which the work is done. For instance, the stu- 
dent of mechanical drawing may use simple and inexpensive 
equipment which would be entirely inadequate in the drafting - 
room of a machine building plant where it is essential to use 
all modern appliances that facilitate the work of the design- 
ing department. When drawings are made on a small scale, 
the drawing paper is attached by means of thumb-tacks to 
a drawing-board which may be supported on a table or desk; 
in most drafting-rooms regular drawing tables are used. These 



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Fig. 27. Drawing-board having Hardwood Cleats on Back to prevent Warping 

drawing tables are practically drawing-boards mounted upon 
some form of stand which is usually arranged to permit ad- 
justing the board. 

The principal requirements of a drawing-board are that the 
top surface be smooth and that at least one of the edges be 
straight, in order to form a suitable guide for the head of the 
T-square. The drawing-board should be made of well-seasoned 
soft pine and be formed of strips glued together edgewise. 
Some boards have end pieces fitted across the end of the 
board by a tongue and groove. The best drawing-boards 
have hard-wood cleats secured to the back of the board to 
prevent warping. The back of a board having these cleats 
is shown in Fig. 27. The screws which hold the cleats pass 
through oblong holes containing metal bushings that fit closely 
under the screw-heads and yet allow the screws to move freely 
in case the board contracts. Grooves are sunk in the board 



INSTRUMENTS AND MATERIALS 



55 



on the under side. These grooves take the transverse strength 
out of the wood to allow it to be controlled by the ledges, 
leaving at the same time its longitudinal strength nearly un- 
impaired. To make the working edge perfectly smooth, allow- 
ing easy movement of the T-square, a strip of ebony is let 




Fig. 28. Drawing-board equipped with Parallel Attachment 

into one end of the board. The strip is sawed apart at about 
every inch to allow for contraction of the board. 

Parallel Attachment for Drawing-boards. — Some drawing- 
boards and tables intended especially for drafting are pro- 
vided with what is known as a " parallel attachment." This 

attachment consists principally of a straightedge (see Fig. 28) 

41 



56 



MECHANICAL DRAWING 



which is used in place of a T-square. The straightedge pro- 
vides means of drawing parallel lines and it is used to locate 
the triangles for drawing vertical lines in the same manner 
as a T-square. The continuous cord to which the ends of the 
straightedge are fastened passes over pulleys at each corner 
of the board. This cord is so arranged that the straightedge 
will maintain its horizontal position when raised or lowered 




Fig. 29. Adjustable Drawing Table 

to any position on the board. The straightedge of the attach- 
ment illustrated may also be set at an angle or be removed 
readily if the use of a T-square is preferred. 

Drafting Tables. — The tables used in drafting-rooms may 
be non-adjustable or adjustable. The adjustable type is now 
used extensively and most of these tables may be adjusted 
for height and also for changing the inclination of the table. 



INSTRUMENTS AND MATERIALS 57 

The method of obtaining the adjustment and the general 
arrangement varies considerably on different mak^s. 

A simple design of drafting table is shown in Fig. 29. This 
is a folding type which may be adjusted to different angles. 
The drawing-board is held at the desired inclination by sup- 
ports which engage notches or teeth formed on the cross- 
pieces of the frame. The drafting table illustrated in Fig. 30 




Fig. 30. Another Type of Adjustable Drawing Table 

has both vertical and angular adjustments. The front and 
side views clearly show the construction. The board is firmly 
secured to cast brackets which are pivoted to the vertical sup- 
porting members. The table is rigidly held in an angular 
position by tightening the hand-lever shown in the view to 
the right. This lever binds against a segment of the support- 



58 MECHANICAL DRAWING 

ing bracket and it is threaded to a long bolt, the head of which 
clamps the bracket on the opposite side. The table is ad- 
justed vertically by the handwheel shown. This wheel is 
mounted on a cross-shaft having pinions at each end, which 
engage teeth cut on the supporting legs. The board is equipped 
with a parallel attachment. 

Drawing Paper. — As most mechanical drawings are first 
drawn in pencil and then traced in ink upon transparent cloth 
or paper, the paper used for the pencil drawing need not be 
as expensive as the paper that would be required for making 
a finished drawing in ink. There are a great many kinds and 
qualities of drawing paper manufactured to suit the great 
variety of uses to which it is put, since drawings vary from 
pencil sketches to the shaded ink drawings used for illustrating 
purposes. In former years, drawing paper was mostly made 
in sheets of certain arbitrary sizes, those in most common 
use being as follows: Medium, 17 by 22 inches; Imperial, 22 
by 30 inches; and Double Elephant, 27 by 40 inches. The 
quality marked " Whatman" is still used when an extra good 
quality is wanted. The thickness of this paper varies accord- 
ing to the dimensions of the sheet; that is, the larger the sheet, 
the thicker it is. Two different surfaces are furnished : smooth, 
or "hot pressed" (marked HP), and rough, or "cold pressed" 
(marked CP), the latter having a considerably roughened or 
"grain" surface. There are many other kinds and qualities 
of drawing paper of all grades, sizes, and prices, and of both 
foreign and domestic manufacture, which are used for mechani- 
cal drawing. Drawing paper in sheets is used to a much 
greater extent in schools and by artists and other free-hand 
draftsmen than by mechanical draftsmen. The reasons for 
this are that very large sizes of sheets are seldom required by 
the former classes of draftsmen, that paper in sheets is more 
conveniently stored in drawers than if it is in the form of 
rolls, and that it is more easily procured in small quantities. 

There are a variety of colors, or rather tints or shades of 
drawing paper from clear white to buff, gray, terra cotta, and 
the natural manila tints. While the artist uses those tints 



INSTRUMENTS AND MATERIALS 59 

which best suit the character of the work, the mechanical 
draftsman is governed by questions of utility. Pure white 
paper is not usually as agreeable to the eyesight as paper 
having a slight cream or ecru tint. A slight tint shows the 
effect of dust less than pure white, and it is not so easily soiled. 
When soiled by being worked upon for a considerable length 
of time, it is usually easier to clean and put in a presentable - 
condition. It is more difficult, however, to trace a penciled 
drawing made on buff or cream-colored paper than it is one 
made on white paper. 

On account of the variety of sizes of sheets needed by the 
mechanical draftsman, paper manufactured in rolls is usually 
preferred, as sheets of any desired dimensions may be readily 
cut from the roll. In many cases quite large construction 
drawings are required, which necessitate sheets as large as 4 
feet wide and 8 to 12 feet in length. For ordinary shop draw- 
ings it has come to be regarded as good practice to adopt 
standard sizes for the sheets or drawings, making the stand- 
ard sheet, say, 24 by 36 inches, and subdividing this into 
half -sheets, 18 by 24 inches; quarter-sheets, 12 by 18 inches; 
and eighths or " sketching" sheets, 9 by 12 inches. If a sheet 
larger than 24 by 36 inches is needed, a " double sheet," 36 
by 48 inches, is used. Sheets larger than this will be used 
only for construction drawings, and no regular dimensions 
are fixed, although the usual width of continuous or roll draw- 
ing paper, 48 inches, determines the width. Roll drawing 
paper can be procured, however, in other widths, these being 
30, 36, 38, 40, 42, 56, 58, and 62 inches. These rolls contain 
from 10 yards of the finer and more expensive grades of paper, 
to 24 and 50 yards for medium grades, and for the cheaper 
grades the rolls are sold by the gross weight, in quantities of 
50, 100, and 150 pounds. 

In the case of paper sold in sheets the prices are fixed at so 
much per sheet, or per quire of 24 sheets. Occasionally the 
price is given per ream of 480 sheets. If sold in rolls of so 
many yards, the price is fixed by the yard; if sold by the 
weight, at so much per pound for whatever the roll may weigh, 



60 MECHANICAL DRAWING 

as the rolls will vary somewhat from the standard weight. 
Continuous or roll drawing paper is made in almost as great 
a variety of qualities, thicknesses, and tints as that which 
comes in sheets. Usually, however, such drawing paper is 
either of pure manila and of the natural color, or of a mixture 
of manila and other materials, and slightly tinted, the tints 
running toward buff and terra cotta. 

Manila Paper. — The surface of manila paper is made 
either very smooth and glossy or with a slight grain. As the 
smooth finish is made by running it between heated calender 
rolls it does not usually lie flat upon the drawing board, but 
has a tendency to buckle or rise in numerous places. A 
smooth surface manila paper is, therefore, not nearly as con- 
venient for doing the pencil work of a drawing as that having 
a slight "grain" or roughness. There is a tendency of the 
pencil to slide over the surface without making a distinct line, 
unless the pressure applied to it is considerable, or unless the 
pencil is much softer than those which the draftsman is in 
the habit of using. Of course, the point of a soft pencil wears 
away much faster than a hard one and, therefore, requires 
more attention to keep it in proper condition. Another diffi- 
culty is that the smooth surface is liable either to stain or to 
become rough under the action of the erasing rubber. It is, 
however, a very strong paper, and its price is moderate. 
Manila drawing paper with a rough or "grain" surface is well 
adapted to either pen or pencil work, and works well under 
the erasing rubber. The slight tint of the surface is an ad- 
vantage to the eyesight of the draftsman. It is generally 
purchased in rolls. 

Eggshell Drawing Paper. — Eggshell drawing paper is one 
of the best papers made for drawings. It withstands a good 
deal of hard usage while being made and much rough usage 
afterward. While it is made in sheets of the usual sizes, it 
is more often obtained in continuous rolls. On account of the 
high price of this paper, these rolls usually contain only 10 
yards each. The surface of this paper, as its name indicates, 
somewhat resembles the surface of an eggshell, except that 



INSTRUMENTS AND MATERIALS 6 1 

the small depressions forming the grain are deeper and quite 
pronounced. The surface is very hard and takes either pencil 
or ink readily, and will stand almost any amount of erasing. 

Mounted Drawing Paper. — Mounted drawing paper is 
drawing paper strengthened by being mounted on cloth, or 
having a backing cloth pasted to it. This process is usually 
confined to the more expensive kinds of white and slightly 
tinted drawing papers, and generally to those in large sheets 
or rolls. Such paper is used for maps and plans that are in- 
tended to withstand much handling and hard usage. Mounted 
drawing paper is used only occasionally for mechanical 
drawings. 

Bristol Board. — Bristol boards are a high quality of card- 
board, made by pasting several sheets of high-grade linen paper 
together so as to form the thickness required. After pasting, 
they are subjected to* a heavy pressure, and present a very 
smooth surface for ink work. The thickness of bristol boards 
is indicated by the number of sheets pasted together to form 
the "board." Hence, they are called 2-sheet, 3-sheet, 4-sheet 
and 5-sheet, the last being the thickest usually made. The 
3-sheet bristol board is specified by the United States Patent 
Office as the proper material to use for patent drawings. 

Cross-section Paper. — Thus far all the drawing papers 
mentioned have been those with absolutely blank surfaces. 
There is, however, a large class of drawing papers on which 
preliminary lines in two directions, at right angles to each 
other, are ruled as a valuable aid to the draftsman in laying 
out his work. These are known as cross-section papers. 
Strictly speaking, cross-section paper is that in which the 
horizontal and vertical ruling is spaced at the same distance 
apart, there being, for example, eight or ten lines to the inch. 
When this ruling is so made that the horizontal spaces of the 
ruling are fewer than the vertical, it is properly called profile 
paper. In this case the vertical ruling is 20, 25, or 30 to the 
inch. Cross-section paper is used for making sketches, dia- 
grams, etc., and for free-hand work; the ruling is of great as- 
sistance in properly proportioning the parts. It is also largely 



62 MECHANICAL DRAWING 

used in the plotting of graphic charts and similar work. It is 
nearly always white. 

Profile paper is used by civil engineers for representing the 
profile or cross-section of grades, cuts, embankments, excava- 
tions and the like. This paper is made in sheets 1 6 by 20 and 
17 by 22 inches, and also in continuous rolls. For the use of 
railway surveyors, the profile paper is made in continuous 
strips, folded between covers in book form; these books are 
called profile books. 

Tracing Paper. — The principal feature of ordinary tracing 
paper (thin, transparent paper) is its cheapness. However, 
it is useful for temporary work or when there is to be very 
little handling of the tracing. So-called "onionskin" paper 
is much used for tracings. While not as transparent as ordi- 
nary tracing paper, it is much stronger. 

Parchment paper may be used as a drawing paper. The 
entire work of the drawing, both pencil and pen work, may 
be done on it, and the drawing may then be used the same as 
an ordinary tracing for producing blueprints. By the use of 
this paper only one drawing is made, instead of a drawing and 
tracing as with the usual methods. 

Tracing Cloth. — Tracing cloth is largely used for regular 
tracings that must be handled a great deal. It is made of 
finely woven and very smooth cloth coated with a prepara- 
tion of Canada balsam to give it a fine surface for the use of 
the pen, and also to render it transparent. One side is smooth 
and glossy while the opposite side has a dull finish. This 
peculiarity is known as "dull back"; the tracing cloth can 
be purchased with a gloss surface on both sides if desired. It 
is possible to use it for a preliminary pencil drawing, to be after- 
ward inked in by working upon the dull side, although this is 
not usually practiced except for simple work in which com- 
paratively few lines are needed. Tracing cloth is nearly 
always sold in continuous rolls, 24 yards long and 30, 36, or 
42 inches wide. 

General Considerations in Selecting Drawing Paper. — In 
selecting a suitable paper for a certain work there are several 



INSTRUMENTS AND MATERIALS 63 

requirements which should be kept in mind, and which may 
be enumerated as follows : The paper should be of such a tint 
as to be agreeable to the eyes of the draftsman. It should be 
possible to stretch the paper readily so that it lies flat on the 
drawing-board. Where much pencil work is to be done, as 
in designing, the paper must stand the frequent use of the 
erasing rubber, and it must take pencil lines easily, even if a 
pencil as hard as 6H is used. If the drawing is to be finished 
in ink, the quality of the paper must be such as to take ink 
readily, without wrinkling where the ink lines are made. The 
thickness and texture of the paper should be uniform over 
the entire surface. A good drawing paper should be of such a 
surface as not to absorb liquids too readily, but not to repel 
them at all. If the surface is repellant, inks or colors are 
liable to rise in small blotches on parts of the ink line and 
leave an insufficient quantity of ink to make a good line at 
other points. While all these qualities will not be likely to 
be found in any one paper, a paper should be selected embody- 
ing as many of them as possible. 

In selecting a proper medium for tracing, the draftsman 
must be guided by the conditions of the work. These condi- 
tions will ordinarily be as follows: For temporary drawings, 
and when but a few blueprints are to be made, ordinary trac- 
ing paper will be used; if the tracing is to be used for a large 
number of blueprints, or for permanent use as a record, it 
should be made on good tracing cloth. If a drawing as well 
as a tracing is wanted quickly, or if the drawing is to be of a 
comparatively temporary nature and considerable pencil 
work with the usual erasures is expected, parchment paper 
may be used. 

Opaque Drafting Fabric. — A drafting fabric which is known 
as "unidraft," differs from any of the other drafting materials 
previously referred to in that it provides for making blue- 
prints directly from the drawing on the fabric, which is the 
original drawing. This fabric is somewhat similar to ordinary 
tracing cloth but it is covered with an opaque surface. In 
making a drawing on this material, the draftsman works with 



64 



MECHANICAL DRAWING 



a pointed steel tool instead of a pencil; this tool is used exactly 
as a pencil would be and scrapes away the opaque surface 
along the lines which make up the drawing. This exposes 
the transparent fabric to permit the passage of light. In 
making blueprints from such drawings, the method of pro- 
cedure is exactly the same as where ordinary tracings are 
used, but as the lines are transparent and the remainder of 
the drawing opaque, the resulting blueprint has blue lines on a 
white background instead of white lines on a blue background. 
The use of this material eliminates the necessity of making 
first a pencil drawing and then a tracing, for the purpose of 




Fig. 31. Enlarged View of Tool used to draw on an Opaque 
Drafting Fabric 

producing blueprints. The surface of the fabric is a dull 
brown which reflects very little light into the draftsman's 
eyes, thus reducing eyestrain. It is possible to make a blue- 
print at any time before the drawing is completed and this is 
often a valuable feature in the case of drawings showing 
general lay-outs. As the surface is a dull brown, it does not 
show dirt, which is a point that will be appreciated by those 
who have had experience in the making of drawings which 
are worked on for a considerable length of time, or with draw- 
ings which are allowed to lie about in places where there is a 
lot of dust and dirt. Another point in favor of "unidraft" 
is that the necessity for sharpening pencils is eliminated be- 
cause the draftsman works with a steel tool which does not 
change its shape. 

It is easy to erase a line made on this fabric, the "erasing'' 
being done with an ordinary writing pen which inks in the 



INSTRUMENTS AND MATERIALS 



65 



line; thus, erasing is merely a matter of replacing the opaque 
surface where it has been taken off by mistake. After the 
erasure has been made, it is possible to redraw any part of 
the line with the steel tool. In the event of it being desired 
to change a full line into a dotted line, this is very easily done 
by simply dotting in the spaces with ink. 

The width of line may be varied by varying the pressure 
on the point of the tool in the same way that the width of a 
pencil line can be varied by varying the pressure. The fabric 
may be worked on with a drafting machine and with the 




Fig. 32. Example of a Drawing made on Opaque Drafting Fabric, 
producing White Lines on Dark Background 

usual drafting instruments without scratching its surface, 
except at those points where it is desired to draw the lines. 

An enlarged view of the steel tool used for drawing the 
straight lines is shown in Fig. 31. Circles are drawn by using 
a bow pencil or compass provided with a steel point. Figure 32 
illustrates how the white lines drawn on this fabric stand out in 
contrast to the dark background. Unidraft fabric is a product 
of the Universal Drafting Machine Co., Cleveland, Ohio. 

Thumb-tacks. — Thumb-tacks are used to fasten drawing 
paper, tracing paper, and tracing cloth to the drawing-board. 
The head, which is the shape of a round disk, is slightly crowned 



66 



MECHANICAL DRAWING 



on the top and has a very thin edge. This permits the 
T-square to slide over the head smoothly. Many thumb- 
tacks have steel points which are attached to brass or Ger- 
man silver heads. Four different sizes having heads varying 
from -^6 to f inch in diameter are shown at A, Fig. 33. The 
J-inch size is extensively used. The tack shown at B has a 
beveled edge. At C is shown an inexpensive tack which is 
stamped from one piece of steel, the point being bent down 





( 


O 


Oi 


f 

A 


\ 


J 

C 


^1 

D 


B 


F 

Mac 


hinery 



Fig. 33. Thumb-tacks for holding Paper or Tracing Cloth to Drawing-board 



from the head of the tack. At D is illustrated what is known 
as the " center-pull thumb-tack.'' This steel tack is turned 
from solid metal. It has a thin head with a knife edge and 
the T-square straightedge rides over the head very easily. 
The special puller E which engages two holes in the head 
makes it easy to pull the tack out straight. At F is shown a 
common type of thumb-tack puller. The end used for pull- 
ing thumb-tacks has a beveled V-shaped groove which can 
be slid under the thumb-tack head. 

Drawing Pencils. — Drawing pencils are usually made of 
hexagonal form to prevent their rolling off an inclined draw- 



INSTRUMENTS AND MATERIALS 67 

ing board. They are made in different degrees of hardness, 
the softer grades usually being indicated by the letter B fol- 
lowing a number and the harder grades by the letter H fol- 
lowing a number. The following is the complete list of the 
degrees of hardness made by some of the well-known manu- 
facturers: 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 
5H, 6H, 7H, 9H. Some manufacturers, however, have a dif- 
ferent system of designating the degree of hardness, but this 
is the method usually employed. 

Many draftsmen use a 5H or 6H pencil on all shades and 
grades of drawing paper. The hardness of lead should be 
varied somewhat for different papers to make the drawing 
visible through the tracing cloth. In most drafting-rooms, 
the papers used have a coarse-grained surface and are either 
white, buff, or cream color. Experience has shown that on 
the white drawing paper a 5H pencil makes lines which may 
be seen readily through the tracing cloth, but on the buff color 
paper a 4H should be used, and on the deeper shades a 3H is 
desirable. Since a much greater pressure can be secured in 
making lines with a lead pencil than with a pencil compass, 
the lead in the compass should always be one degree softer 
than the pencil. If this method is followed, all drawings will 
have good black lines, which will lessen the strain on the eyes, 
especially in making tracings. 

Sharpening Drawing Pencils. — In order to produce neat 
pencil drawings, it is necessary to keep the pencils properly 
sharpened. There are two methods of sharpening lead pencils 
for mechanical drawings: the round or conical point and the 
flat or chisel point. The rounded point shown at A and B, 
Fig. 34, is always used for lettering and dimensioning when a 
pencil is used. Some draftsmen use the round point for all 
ordinary drawing and employ the chisel point shown at C and 
D, only when very fine lines are required. The chisel point, 
being thinner, can be used to produce more lines without re- 
sharpening than the conical point can. In sharpening the 
pencil, the lead is given either a conical or chisel point by 
rubbing on sandpaper or a file. 



68 



MECHANICAL DRAWING 



Ink Used by Draftsmen. — A good India ink should be 
used for all mechanical drawings. It can be purchased in 
small bottles equipped with a quill filler attached to the cork. 
A good quality of ink will flow freely and dry quickly. Water- 
proof drawing ink is to be preferred in most cases. The 
bottle must be kept corked to prevent evaporation. If the 
ink becomes too thick and does not flow freely, it may be 
diluted with a few drops of water. Prepared drawing inks of 
various colors may be obtained, but they find limited use in 



® © 



Machinery 



Fig. 34. Methods of sharpening Pencils 

ordinary drafting-room work. Red ink is sometimes used in 
making center fines and dimension lines on tracings, but this 
is not the general practice and, if followed, the blueprinting 
must be carefully done in order to have the lines show distinctly. 

Ink produced from India or Chinese sticks was employed 
by draftsmen previous to the introduction of prepared ink. 
The Chinese stick or the India ink is prepared by grinding 
with a small quantity of water. 

Erasing Pencil and Ink Lines. — Draftsmen should have 
the proper equipment for making erasures on paper or trac- 



INSTRUMENTS AND MATERIALS 69 

ing cloth, as mistakes are unavoidable. The usual form of 
pencil eraser is made from soft pliable rubber.. The rubber 
ink eraser is made from rubber which has very fine sand or 
pumice stone incorporated into it. When it is desired to 
restrict the erasure to a limited area, an erasing shield made 
from a thin plate of some material such as steel, brass, cellu- 
loid, or paper should be used. It has openings or holes cut 
in it of various shapes and sizes. When this shield is used, 
an opening of the required size and shape is placed over the 
portion to be erased, and the rubber eraser applied to the 
opening. A steel erasing knife is also used to erase ink lines 
from paper or tracing cloth. An oilstone should be used to 
sharpen the edges when they become dull. When using the 
erasing knife care should be taken to keep the blade perpen- 
dicular to the paper or tracing cloth and only a very light 
pressure should be exerted on the drawing. The action of the 
cutting edge should be confined to the ink alone and the sur- 
face of the paper or tracing cloth should be scratched as little 
as possible. The erasing knife is particularly useful in erasing 
heavy lines and, if it is used with care, good results may be 
obtained. 

The usual practice in erasing heavy ink lines is to use the 
erasing knife until the line has nearly disappeared, then an 
ordinary rubber ink eraser and finally a soft rubber pencil 
eraser. After an ink erasure has been made, the surface of 
the paper or tracing cloth should be rubbed with a piece of 
soapstone. The soapstone resizes the surface and prevents 
the ink from penetrating the paper or cloth, thus causing blots 
when drawing ink lines over the erased surface. If heavy lines 
are required to be drawn over an erased surface it is usually 
a good plan to produce the required weight of line by drawing 
a series of very fine lines instead of one heavy line, allowing 
the ink to dry each time, before drawing the succeeding line. 



CHAPTER IV 

HOW DESIGNS ARE ORIGINATED, AND PROCEDURE IN 
MAKING DRAWINGS 

The work done in the drafting-room includes the origina- 
tion and improvement of mechanical devices, and the making 
of drawings that will enable these various forms of mechanisms 
to be constructed. The work of origination may be, and 
often is, due to the combined efforts of several men, or it may 
be confined largely or entirely to one man. Frequently the 
basic idea or principle is thought out by a designer, chief 
draftsman, or sometimes by a shop foreman or superintendent. 
This idea or conception of a new device is, according to a com- 
mon method of procedure, first represented by a sketch which 
may be simply a crude drawing made free-hand. Then the 
next step is to make an accurate drawing in order to obtain 
a better idea of the relation of different parts, and finally, if 
the design is approved, working drawings are made which 
are the ones needed in actually constructing the device. Those 
who originate new forms of mechanisms may consider their 
time too valuable for making drawings other than the pre- 
liminary sketches such as are used to show the draftsman 
what the general requirements are. The draftsman, however, 
who makes drawings that agree in regard to the original 
features with sketches furnished him, is often relied upon to 
do more or less original work, especially in developing and 
perfecting the various details of the design. 

Preliminary Work in Designing a New Mechanism. — The 
work of developing the design of any mechanical device de- 
pends very largely upon the nature of the device and its pur- 
pose. Some forms of mechanical apparatus are simple as 
far as their operation is concerned, and little time or thought 
is expended in deciding how the parts are to be arranged, but 

70 



GENERAL PROCEDURE 71 

perhaps the proportioning of these parts to withstand safely 
the stresses to which they will be subjected in actual service, 
involves careful calculations. A hoisting mechanism, con- 
sisting of a train of gears and with shafts subjected to bending 
and torsional stresses, illustrates this general type of apparatus. 

Then there is another general group of mechanical devices 
which differs from the class just referred to, in that few, if 
any, calculations for stress are required, because the parts are 
not subjected to loads or stresses worth considering; neverthe- 
less, the origination of the design may be difficult. Take, for 
example, the case of a machine which must be complex in 
order to perform mechanically some difficult operation. The 
adding machine, for instance, is a very complicated mechanism 
which was exceedingly difficult to design because of the intri- 
cate mechanical actions required. Very little power, however, 
is needed for operating a machine of this kind, and the stresses 
on the different parts are small, so that the big problems con- 
fronting the designer were those pertaining to mechanical 
action. 

In the design of apparatus for generating power, another 
class of problems is encountered. For instance, in the design 
of steam engines, an essential part of the work is to develop 
an engine which will be economical in operation, or one using 
the least possible amount of steam per horsepower. If pump- 
ing machinery, air compressors, etc., are considered, it is 
apparent that physical laws which affect the operation of such 
equipment must play an important part in the development 
of the designs. 

Another branch of designing work which is quite different 
from those mentioned has to do with the design of various 
kinds of tools used in machine building plants. Such tools 
include the general group known as machine tools and also 
the smaller equipment used in conjunction with them. For 
instance, various classes of machine tools, such as turret lathes, 
screw machines, drilling, milling, and grinding machines, etc., 
require special cutting tools and work-supporting devices. 
The designer of such tools must necessarily understand manu- 



72 MECHANICAL DRAWING 

facturing methods, and he is often an expert on machine shop 
practice; in fact, draftsmen or designers specializing in this 
work are frequently "graduates" of the shop. 

Now it is quite evident that a designer cannot be an expert 
in all branches of work, because the field is so large and ad- 
vances so rapidly that no one mind can grasp all of the essen- 
tial facts. The natural result has been the development of 
specialists in machine design just as there are specialists in 
surgery. For instance, there are designers who work exclu- 
sively on some general class of machinery, such as special 
automatic machinery; power plant equipment; jigs, fixtures, 
and gages; machine tools; and other general groups which 
might be mentioned. The draftsman who is equally com- 
petent in any plant regardless of the nature of the work is 
one who does not originate, but merely copies on paper or 
tracing cloth what the specialist has planned for him. 

To be able to draw and understand drawings is like being 
able to read a language; it is merely a beginning for the man 
who is primarily interested in machine design. Many stu- 
dents of mechanical drawing do not understand what is in- 
volved when they begin to study this subject, and that is 
why an attempt has been made at the beginning of this book 
to show clearly the relation between the mere drawing of lines 
on paper and the much greater and more valuable work of 
actually creating new or improved forms of mechanical appa- 
ratus. The making of the drawing, however, is an essential 
step, but drawing alone should be regarded as merely one 
part of the draftsman's stock in trade. Reference has already 
been made to some of the other subjects that he should 
understand. 

What Mechanical Drawings are Based on. — When a 
mechanical drawing is to be made, the draftsman usually 
has a certain amount of information and data or figures given 
him as a basis or guide in making the drawing. This infor- 
mation, as before mentioned, may be in the form of a sketch 
on which important dimensions are marked, especially if the 
design is different from any that has preceded it. If some 



GENERAL PROCEDURE 73 

existing design is to be improved, the draftsman may work 
from drawings of it, making whatever changes have been 
suggested or those that seem desirable. In some cases, a draw- 
ing is required of a machine or tool either because the original 
drawings are not available, or because the device has been 
constructed without proper drawings. A common method of 
procedure in this case is first to make one or more sketches of 
the device and then use these sketches (which should contain 
all important dimensions) when making an accurate drawing. 

A great many drawings are made by experienced draftsmen 
without any preliminary sketches or any information other 
than an order from the chief draftsman or some other official 
for the drawing of a device adapted for a certain purpose. 
For instance, in the design of drill jigs, milling fixtures, etc., 
a competent draftsman may simply know what kind of a 
piece is to be held in the jig or fixture and the nature of the 
machining operation. He then proceeds to work out a de- 
sign which conforms to approved practice in the design of such 
equipment. A draftsman capable of such work is a designer 
in this particular field, although he may not be competent to 
do work requiring a general knowledge of mechanical engineer- 
ing subjects. 

Uses of Different Classes of Drawings. — The draftsman 
should understand clearly the uses of different classes of draw- 
ings and their relation to one another, because there are several 
distinct types of drawings which serve different purposes. 
Most of the mechanical devices shown on drawings are com- 
posed of different parts which are assembled to form the com- 
plete mechanism. Now if the number of parts is relatively 
small, it is evident that the shape and size of each part might 
be clearly shown on a single drawing consisting possibly of 
two or three views; but when a mechanism is more complicated 
and is formed of many different shafts, levers, gears, etc., if 
an attempt were made to show all of these on one drawing, 
the result would be a mass of lines which, in some instances, 
would be so closely spaced and interwoven that the drawing 
could not be used as a guide when constructing the mechanism. 



74 



MECHANICAL DRAWING 



It is frequently necessary, therefore, to make separate draw- 
ings which show the details more clearly than a complete 
drawing of the entire machine. There are then general or 
assembly drawings, and detail drawings. 

Classes of Assembly Drawings. — An assembly drawing 
may show a machine in outline merely to illustrate its general 
arrangement (the side elevation, Fig. i, illustrates this type 




Fig. 1. 



Side Elevation of a Nut-planing Machine 
Drawing 



An Example of an Outline 



of drawing), or the assembly drawing of a comparatively 
simple mechanism may contain all the necessary dimensions, 
in which case it is a " working drawing," the latter term being 
applicable to any drawing which contains the necessary dimen- 
sions and instructions for constructing whatever the drawing 
represents. 

When the design of a new machine is being developed, an 
assembly drawing is commonly made first, because it shows 



GENERAL PROCEDURE 75 

the relation between different parts and assists the designer 
in working out his plans. This assembly drawing, which is a 
preliminary design, is frequently made full size unless the 
mechanism is too large to permit of doing this. There is 
often an advantage in having this first drawing full size, 
especially when it is important for the designer to be able 
to see the various details clearly, and particularly when there 
are numerous parts which are likely to interfere when 
the mechanism is in operation, owing to small clearance spaces. 
This preliminary assembly drawing is not always confined to 
one machine, but it may show several different kinds of appa- 
ratus forming a complete plant and possibly the necessary 
piping, etc. 

After the assembly drawing is finished, the drawings of 
details are made, and then an assembly drawing may be re- 
quired in the erecting shop to show the machinist who as- 
sembles various parts just how they are arranged. The pre- 
liminary design, which is drawn in pencil, may be traced and 
blueprints made for use in the shop, but frequently another 
assembly drawing is made to a smaller scale, so that it may 
be placed on a sheet of standard size. This would be done 
if the preliminary design were too large for actual use in the 
shop. If this second assembly drawing is made by referring 
to the dimensions on the various detail drawings, errors in the 
dimensions of the details may be discovered since the different 
units forming the complete drawing will not go together 
properly. An assembly drawing for use in the erecting shop 
usually contains the principal dimensions, as, for example, 
the center-to-center distance between important shafts and 
whatever general dimensions might be needed in properly 
assembling the different units forming the complete machine. 
When making a drawing of this kind, the draftsman should not 
necessarily show by dotted lines all concealed parts, because 
frequently they would be useless and simply complicate the 
drawing and make it confusing. In some cases, it is necessary 
to show by dotted lines the location of certain important de- 
tails for the guidance of men in the assembling department. 



76 MECHANICAL DRAWING 

These details might be within the bed of the machine or in 
some other concealed location, and perhaps the use of dotted 
lines would enable their position to be shown very clearly, 
thus avoiding a separate view of the entire machine. 

When the assembly drawing is merely an outline of the 
machine, it may be intended as the catalogue illustration or 
to show the purchaser of the machine how it should be set 
up. Some drawings of this kind are used to show the location 
of foundation bolts, so that the foundation can be built before 
the machine arrives. Other outline drawings are used to 
show the relation between the machine and the overhead 
works, as in the case of a belt-driven grinder. Another pur- 
pose of an outline assembly drawing is to enable the user of 
a machine to identify different parts. For instance, if the 
separate parts or units of the machine are numbered on the 
outline drawing, these numbers can be used when ordering 
new parts instead of attempting to name or describe them. 
These numbered drawings are sometimes found in manu- 
facturers' catalogues. 

When Detail Drawings are Required. — Whenever a 
mechanical device is formed of so many parts that a drawing 
showing all of them assembled would be confusing, separate 
working drawings of different important units or details are 
made. A detail working drawing may show only one casting, 
forging, or other piece, or the detail drawing may represent 
a certain group of parts which form some unit in the com- 
plete machine. For instance, a separate drawing might be 
made of a gear-box for changing the speed of a machine tool. 
By making a separate drawing of a unit of this kind, different 
views of this particular unit can be shown and to a larger 
scale than would be possible on a drawing which showed the 
complete machine. When the different views on the draw- 
ing of the gear-box are provided with all necessary dimen- 
sions, they show the men in the shop just how that particular 
gear-box is to be constructed. This is an example of a detail 
working drawing. Frequently several small details are shown 
on one sheet, even though these parts may not belong together 



GENERAL PROCEDURE 77 

when assembled, but a separate sheet for each independent detail 
or group of parts forming the independent unit is desirable. 

Sizes of Drawings. — Mechanical drawings may be made 
to full or "life size" or they may be drawn to a reduced scale. 
While it is necessary to make drawings large enough so that 
all of the details may be seen easily, it would obviously be 
impracticable to draw all kinds of machinery full size because, 
even in the case of machines of moderate size, very large 
and unwieldy drawings would be the result. These large 
drawings are not only inconvenient to handle in the shop, 
but it is an awkward procedure to make them, merely because 
of their size and the difficulty of reaching to various parts of 
large drawings across the drawing-board. 

It is customary in different drafting-rooms to adopt certain 
standard sizes (or widths and lengths) for sheets of drawing 
paper or tracings, and then the drawing is made to whatever 
scale the size of sheet will permit. While there is no universal 
standard among manufacturers governing the sizes of draw- 
ings, the following sizes are quite common: 24 by 36 inches; 
18 by 24 inches; 12 by 18 inches; 9 by 12 inches. 

Sheets of these sizes have been adopted because they can 
be cut from commercial rolls of drawing paper, tracing cloth, 
and blueprint paper with little waste. It will be noted that 
when the 24- by 36-inch sheet is cut in half, it forms two 18- 
by 24-inch sheets, and the latter forms two 12- by 18-inch 
sheets. The smallest size given is sometimes called a "sketch- 
ing sheet," and it is made by cutting a 12- by 18-inch sheet 
in half. The larger sheets are intended for the assembly 
drawings or for detail drawings of complicated parts which 
must be drawn to a fairly large scale in order to show all of 
the small details. Many of the smaller details may be drawn 
full size, but the larger details and the assembly drawings are 
almost invariably drawn to a reduced scale. For instance, a 
large detail drawing may be one half or one fourth the actual 
size of the detail represented, and it may be necessary to re- 
duce the assembly drawing to one eighth, one twelfth, or even 
one sixteenth of the actual size. 



78 MECHANICAL DRAWING 

At one of the large automobile plants, five different sizes of 
sheets are used. The first is the standard letter size, 8f x n 
inches. All other sizes are developed from the first size; that 
is, all are multiples of 8 J by n inches, or of one of those 
dimensions. No. i sheet is always made with a wide blank 
margin at the left on the short dimension, and a narrower 
uniform blank margin on the other three sides. The drawing 
or sketch is placed on the paper to read lengthwise of the 
sheet. Sheet No. 2 is 11 by 17 inches, with a wide blank 
margin at the left on the short dimension, and a narrower 
blank margin on the other three sides. The drawing is made 
to read lengthwise of the sheet. Sheet No. 3 is 17 by 22 
inches, sheet No. 4, 22 by 34 inches, and sheet No. 5, 34 by 
44 inches, with the same arrangement of margins. 

Another plan is to use only sheet No. 2, which is n by 17 
inches, drawing large objects to a reduced scale on this size 
sheet, with a statement of the scale used on the sheet margin, 
so that the drawing will be readily understood. This method 
would do away with the necessity of having different sized 
sheets, and all of the books for shop use would be made up 
with one size of sheet. In making drawings of such parts as 
crankcases, cylinders, etc., they would be drawn to a reduced 
scale with only the machining dimensions placed on the sheet. 
Pattern drawings would be made full-size, and would be kept 
in the drafting-room for factory reference. One or two large 
drawings might also be placed in the tool-room or at some 
other convenient point, for reference after a job has been 
started. All of the drawings in regular use, however, would 
be 11 by 17 inches. 

How to Use a Draftsman's Scale. — When drawings are 
made full-size, a scale is used which is graduated in inches 
and subdivisions of an inch, in the usual manner. This scale 
is also used for half-size drawings or those drawn to a scale 
of 6 inches = 1 foot. The half-inch divisions on the scale 
are then considered the same as inches, and the sixteenth divi- 
sions correspond to eighths of an inch on the half-size drawing. 
If a half-size drawing is too large to go on a standard sheet 



GENERAL PROCEDURE 



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80 MECHANICAL DRAWING 

and a still greater reduction of size is required, then a scale 
having special graduations is used. The reduced scales gener- 
ally used on mechanical drawings are as follows: 
Scale of 6 inches = i foot (J size) 
Scale of 3 inches = i foot (J size) 
Scale of i J inch = i foot (J size) 
Scale of i inch = i foot (yV size) 
Scale of f inch = i foot (xe size) 
A draftsman's scale which has four sets of graduations, 
representing f , J, ij, and 3 inches to the foot, is illustrated 
at A, Fig. 2. The graduations representing ij and 3 inches 
to the foot are on one edge and those for | and f inch to the 
foot, on the other edge. The method of reading and using 
one of these scales will be explained considering first the scale 
of 3 inches to the foot. This scale is on the upper edge at the 
left, as seen in this particular illustration. A length of 3 
inches along this edge is divided into twelve equal parts repre- 
senting inches and each of these inch divisions is further 
divided into eighths. This 3-inch section of the scale is con- 
sidered the same as though it were 1 foot long, since it repre- 
sents a length of 1 foot on the reduced scale of the drawing. 
It will be noted that the zero mark is at the right-hand end of 
the scale instead of being at the left, and that the numbers 3, 
6, and 9, representing inches, read from the zero mark to the 
left. The divisions to the right of the zero mark, however, 
which represent feet, read from left to right, the readings in 
each case being away from the zero mark. With this arrange- 
ment, the number of feet and inches can be read directly. 
The arrow a is equivalent to a measurement of 1 foot 3 inches, 
and illustrates how the scale is read. Incidentally a scale 
divided in this way is known as an " open-divided scale" to 
distinguish it from the " full-divided " or "chain" scales used 
by civil engineers, which have equal divisions and subdivi- 
sions extending along the whole length of the page, so that 
only one set of graduations can be placed along one edge. 

The scale of i| inch to the foot starts at the opposite end 
of the same edge, and the numbers representing feet are placed 



GENERAL PROCEDURE 8 1 

between those for the 3 -inch or i-size scale. The arrow b is 
equivalent to a measurement of 2 feet 9 inches, as will be 
seen by examining the scale graduations. When two sets of 
divisions or two different scales are placed on one edge as in 
this case, one must be double the other. For instance, ij 
inches and 3 inches to the foot may be placed together and f 
and I inch per foot are on opposite edge of the scale illus- 
trated at A, Fig. 2. The meaning of the different numbers 
opposite the graduations on the lower edge of the scale will 



Machinery 



Fig. 3. Different Kinds of Lines used on Drawings 

now be apparent, since the arrangement is similar to that 
described. 

The scale illustrated at B has only one set of graduations 
on each edge, the scale on one side being 1 inch to the foot 
and on the other, \ inch to the foot. The arrow c is equiva- 
lent to a reading of 7 feet 6 inches on the inch scale and the 
arrow d, a reading of 12 feet on the half -inch scale. Still an- 
other type of scale is shown at C. Both edges of this scale are 
graduated to 1 inch to the foot. The advantage of using a 
scale of this kind is that it is not necessary to turn it around 
in order to bring the right scale into position, as would often 
be the case with the scale shown at A, which has four sets of 
divisions. 

Different Kinds of Lines. — Mechanical drawings are com- 
posed of lines which vary both in form and width, some being 



82 MECHANICAL DRAWING 

full or unbroken, while others are dotted or have some other 
form. If all the lines were alike, there would not be sufficient 
contrast between different parts represented by the drawing, 
and it might be difficult to understand fully the arrangement 
of a mechanism. Since clearness is vital in mechanical draw- 
ings, several different kinds of lines are used to assist the 
draftsman to make a drawing which can be more easily read 
or understood. Unfortunately, there is no universal standard 
governing the kinds of lines to use on mechanical drawings. 
While the visible outline of an object is always represented by 
full or unbroken lines and concealed parts by dotted lines, 
practice varies in regard to the dimension lines and some of 
the others. 

Lines in Common Use. — The lines illustrated in Fig. 3 
are in common use and conform with approved drafting prac- 
tice. The width of the full line A for representing all visible 
details varies somewhat according to the size and purpose of 
the drawing. Such lines on a tracing might be 3V inch wide, 
but perhaps not more than half this width if the ink lines 
were drawn directly on the paper, as is sometimes done. The 
dotted line B, which is used to represent concealed parts of 
surfaces, is usually formed of a series of short dashes about f 
inch long, separated by short spaces, as shown in the illustra- 
tion. When drawing with these dotted or broken lines, 
the spacing is governed entirely by the eye or by judgment, 
no attempt being made to make the dashes and intervening 
spaces conform to given lengths. 

A center line is shown at C. This is another form of line 
which may be considered standard, as it is used by practically 
all mechanical draftsmen. The dimension line D is used to 
show how far and from what points a given dimension extends. 
This is a full line, but much lighter than the visible outline A. 
A space should always be left approximately in the center of 
the dimension line, to receive the figure representing the 
dimension. The arrow-heads at each end of the dimension 
line should preferably be made about as shown in the illus- 
tration, or with the lines forming the arrow-head placed rather 



GENERAL PROCEDURE 83 

close to the dimension line. Some draftsmen make arrow- 
heads which flare out considerably and present a very unsightly 
appearance. While this is a minor point, if a drawing con- 
tains numerous dimensions and the arrow-heads are poorly 
made, the general appearance of the drawing will be greatly 
impaired. 

The extension or reference line E, Fig. 3, is formed of rather 
long dashes and is used to mark the limits of a dimension, as 
shown by the illustrations in different chapters. One of these 
extension lines is placed at each end of the dimension line in 
order to show exactly where on the drawing the dimension 
applies; that is the extension lines are placed directly oppo- 
site the surfaces between which the dimension is given and the 
dimension line is drawn between them. The arrow-heads of 
the dimension line come into contact with the extension lines, 
thus showing clearly where the dimension applies. A small 
space should be left between the extension lines and the lines 
of the drawing proper. Short extension lines are simply one 
continuous line, and in some drafting-rooms all extension lines 
are continuous and the same as the dimension lines in width. 

These extension or reference lines are also called " wit- 
ness" lines in some drafting-rooms. They are not always 
used in conjunction with dimension lines, as this is not 
necessary, because the dimension lines frequently extend 
directly between center lines or the lines of the drawing itself. 
Some draftsmen use light unbroken extension lines and a 
form similar to that shown at E for the dimension lines. The 
object of using broken lines for the dimensions is to secure a 
greater contrast between these lines and the lines of the draw- 
ing proper. It is more convenient, however, to draw the 
unbroken dimension lines, and when they are made lighter 
than the outlines of the drawing, there is no difficulty due to 
lack of contrast. Still another variation in practice is repre- 
sented by the use of dimension lines similar to the form shown 
at E, except that the dashes are separated by double dots. 
It would be desirable if these differences in practice did not 
exist and a universal standard were adopted, but since this 



84 MECHANICAL DRAWING 

is not the case, the draftsman must be governed by standards 
of the plant or drafting-room where he is employed. The 
methods of using the different lines referred to are shown in 
illustrations found in various parts of the book. 

Shade Lines on Drawings. — The working drawings used 
in machine building plants are almost invariably unshaded, 
but some other drawings have shade lines in order to make 
the object appear in relief or stand out from the paper. The 
illustrations in technical periodicals are often shaded, the idea 
being, in many cases, to show the form of the object as clearly 
as possible in a single view. All Patent Office drawings are 
shaded and sometimes shading is applied to assembly draw- 
ings of machines, particularly when they are merely intended 
for purposes of illustration. As draftsmen may be required 
to make these shaded drawings, the methods of shading will 
be explained. 

One method of shading is simply by using light and heavy 
lines to represent the outline of the object. In order to secure 
a more pronounced shading effect, closely spaced parallel 
lines are sometimes used to make curved surfaces appear 
curved, as on a wood engraving. The first method is illus- 
trated by the diagrams in Fig. 4. As will be seen by referring 
to this illustration, heavy shade lines usually represent the 
bottom and right-hand edges, it being assumed that the light 
strikes the paper from the upper left-hand corner at an angle 
of 45 degrees. It is easy to determine which surface is on 
the shaded side by placing a 4 5 -degree triangle upon the 
T-square and assuming the hypotenuse to be the ray of light. 

When shade lines are used, each view is shaded inde- 
pendently of the others, but the lower and right-hand sides 
are shaded in all the views. The shade line should preferably 
be drawn outside the surface that it bounds. Dotted lines 
are never shaded, neither is a line common to two surfaces 
when both surfaces are visible. By referring to Fig. 4, it will 
be noted that shading for a hole is on the upper left-hand side. 
A boss projecting above the surface would be shaded on the 
lower right-hand side, and then it would not be mistaken for 



GENERAL PROCEDURE 



85 



a hole. At A is a square block, hollow in the center. The 
outer shade lines show that the block is raised above the sur- 
face of the paper, and the location of the inner lines shows 
that the center of the block is hollow. At B is shown how 
the block would be shaded if at an angle of 45 degrees. Since 
the projection of a ray of light is supposed to be at 45 degrees, 





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Machinery 



Fig. 4. Diagrams illustrating the Use of Shade Lines 

there is no logical reason why the lines parallel to cc should not 
be shaded instead of cc; but the figure looks well as drawn. 
At C is the shading for a hexagonal prism, and at D, for a 
hollow cylinder. The shading on the top view of D starts 
at the 4 5 -degree line ab, gradually increases, and then di- 
minishes to nothing when it again reaches the line. At E are 



86 



MECHANICAL DRAWING 



two blocks d and e of the same size. No shade line would be 
drawn between them; but at F, where blocks at / and g are 
of different thicknesses, the shade line would be necessary. 
At G the block h is recessed for the cylinder j. It may be 
shaded as shown, although some draftsmen might prefer to 
leave off the shade line k in the lower view. 

The short section of pipe shown in Fig. 5 has been shaded 
by means of closely spaced parallel lines, which gradually 
fade out, thus making the pipe and fitting appear cylindrical. 
This method of shading is often seen on Patent Office drawings. 







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Machinery 



Fig. 5. Another Method of Shading, intended to give the Pipe and Fitting a 
Cylindrical Appearance 



Making the Pencil Drawing. — The making of the pencil 
drawing, which is usually traced afterward on transparent 
cloth, is a very important part of the draftsman's work, be- 
cause it is an actual representation of the design and must be 
drawn quite accurately or in the proper proportions. The 
tracing, which is made later, is simply a copy of the pencil 
drawing and making the tracing requires much less experience 
and knowledge than making the pencil drawing. While this 
pencil drawing should be fairly accurate, no attempt is made 
to have lines terminate exactly at intersecting points merely 
to secure neatness of appearance. In fact, pencil drawings 
often have quite a "sketchy" appearance, even though the 
various lines are located accurately. 



GENERAL PROCEDURE 87 

The first question to be decided is the scale or size of the 
drawing. Usually the size of the sheet must conform to one 
of the standards adopted by the plant for which the drawing 
is to be made; hence, the scale of the drawing must be 
selected accordingly. If a half-size drawing is a little too large, 
a scale of 3 inches to the foot would be used instead, and if 
the latter were still too large, the drawing would usually be 
made to i| inch per foot. In order to locate the different 
views to the best advantage, it is frequently advisable to make 
a very light free-hand outline sketch which conforms approxi- 
mately to the amount of space each view will require. The 
work is then started by drawing the most important lines of 
the principal view or the view which will be of the greatest 
assistance in locating lines on the other views. The principal 
view may be denned as the one which represents the charac- 
teristic shape of the part to be drawn. The center lines are 
drawn first, because it is the general practice to lay off other 
important lines from the center lines, which may be considered 
as the foundation or framework of the drawing. This drawing 
of the center lines first applies to any symmetrical part. It is 
essential to make the pencil lines heavy enough so that they 
can be seen easily through the tracing cloth. The pencil 
drawings should not be made too neatly as this is a waste 
of time. There is no reason why the draftsman should take 
pains with the lettering, cross-sectioning, and the general 
appearance of a pencil drawing. The cross-section lines may 
be drawn free-hand, the neat finishing touches being left to 
the tracer. 

Making Tracings. — When a tracing of a pencil drawing is 
to be made, a sheet of tracing cloth, which corresponds 
approximately in width and length to the standard size sheet, 
is placed over the pencil drawing and is held in position by 
thumb-tacks. The tracing cloth is glazed or smooth on one 
side and dull on the other. The tracing may be made on 
either side, but it is the general practice to use the unglazed 
side, because it takes ink more readily than the smooth glossy 
side. Moreover, pencil fines may be drawn more readily on 

6l 



88 MECHANICAL DRAWING 

the dull side in case this is necessary. When a fine neat trac- 
ing is desired, the glazed side should preferably be used, as 
the lines are somewhat sharper when the ink is* applied to a 
smooth surface. The glazed side also collects less dust and 
ink lines may be more easily erased from it. 

The cloth should be stretched evenly over the surface of 
the pencil drawing and the surface of the cloth should be 
rubbed with some pulverized chalk, soapstone, or one of the 
special preparations used for this purpose, as this causes ink 
to flow more readily upon the surface of the cloth. After 
the chalk or other substance has been rubbed in lightly with 
a dry rag, the tracing cloth is wiped clean and then the ink 
lines are drawn. 

Small tracings are held by a thumb-tack in each corner, but 
large tracings should preferably be held by eight tacks. If 
the following procedure is adopted, the cloth may easily and 
quickly be placed on the drawing-board without wrinkling: 
Put the first tack in the middle of the top margin of the cloth. 
Then smooth the cloth with the palm of the hand downward 
to the middle of its bottom margin, and fasten it there with 
a tack. This makes the cloth taut vertically through the 
middle. Now, smooth out from the center to the middle of 
one of the side margins, fasten with a tack at this point, and 
repeat for the other side. The cloth is now tacked at the 
middle of each of its edges. Finally, smooth out the cloth 
from the center to each corner and tack there. As the cloth 
is always smoothed from the center outward, no wrinkle or 
fullness is left. 

Order in which Ink Lines Should be Drawn. — The ink 
lines on a tracing should not be drawn in haphazard fashion, 
because by proceeding in an orderly manner the tracing can 
be made more rapidly and easily. As a general rule, the 
center lines are drawn first, and then the circles and arcs such 
as are used to represent the fillets in corners, etc. The reason 
for drawing the circles and arcs before the straight lines is 
that ordinarily it is easier to join straight fines to the curved 
lines neatly than to follow the reverse order. The horizontal 



GENERAL PROCEDURE 89 

straight lines are next drawn, beginning at the top of the 
drawing and inking in the various lines as the T-square is 
moved toward the bottom of the drawing. In this way, the 
T-square is prevented from sliding over the lines before the 
ink has had time to dry. The next step is to draw the ver- 
tical lines, which is generally done by guiding the ruling pen 
with a triangle held against the T-square. When drawing 
these vertical lines, the order should be from the left side of 
the drawing to the right. The dimension and extension lines 
are next drawn, and then the section lines, provided any part 
of the drawing is shown in section. As the dimensioning of 
drawings and the use of sections are very important features 
of drafting practice, these subjects will be dealt with sepa- 
rately in a following chapter. 

Tracing cloth is affected considerably by atmospheric 
changes, and whenever possible, the entire drawing should be 
finished on the same day on which it is started. If this can- 
not be done, it is advisable to finish one of the views on the 
tracing instead of partially finishing all the views. 

The ink lines which form the drawing or tracing proper 
may all be of uniform width or they may vary to secure a 
shaded effect, as explained previously in the paragraph headed 
" Shade Lines on Drawings." Whether shading is done or 
not, all fines should be heavy enough to reproduce clearly on 
the blueprint. Perhaps the best way for the beginner to 
obtain an idea of line width is to examine existing tracings 
and compare them with their blueprints. After the different 
views on the tracing have been finished, any explanatory 
notes that may be required are added and also such features 
as the border line and title. When pencil drawings are made 
on thin paper, which is a rapid method sometimes employed, 
care should be taken to use a soft pencil and make heavy 
clean-cut lines that will reproduce well on the blueprint. 

Using Bond Paper instead of Tracing Cloth. — Thin bond 
paper is in many respects better than tracing cloth for draw- 
ings to be blueprinted. It permits the making of a neat look- 
ing drawing, pencil drawings can easily be made on it, a heavy 



90 MECHANICAL DRAWING 

pencil drawing will blueprint nicely from it, and it is cheaper 
than cloth. Besides, tracing cloth does not lie in drawers as 
well as bond paper. When tracings are folded and creased 
across the lines of the drawing this shows on the blueprint. 
There is no trouble on this score with bond paper, and if for 
no other reason than this many have decided in its favor. 
Drawings can be inked more accurately by inking the original 
pencil drawing than by tracing, and it can be done more 
rapidly, which is another advantage for bond paper. 

In many drafting-rooms, with the exception of drawings 
that must be repeatedly and frequently blueprinted, they 
make no tracings. Such as they make are made largely for 
the reason that they blueprint more rapidly. Another reason 
for tracing is that, if the original drawing was used to blue- 
print from too often, it would soon become worn out and 
unfit to make another copy from without much labor. For 
standard erecting plans, etc., it is preferable to make tracings 
on cloth and keep the original carefully, as in the course of 
years it is likely to need alterations. 

From smooth and semi-transparent drawing paper one can 
get a first-class blueprint in about two and one-fourth times 
the number of minutes required for tracing on cloth. The 
drawing is laid out in pencil, then the useless lines are erased 
with a piece of "artgum," which leaves the surface in good 
condition for inking. When the drawing is inked it is done; 
there is no tracing to be made. When making a drawing 
upon which a great deal of time is to be spent such as the 
design of a new machine, the paper is dampened and the 
edges are glued to the drawing-board. When dry it presents 
a surface which is smooth and which is not affected by any 
atmospheric changes and moisture which buckle and wrinkle 
any drawing paper under ordinary conditions. 

Making Changes on Bond Paper Drawings. — Changes on 
bond paper drawings can be made much more easily than in 
the case of tracing cloth. The pieces or parts to be changed 
are cut out and a new piece is pasted in and redrawn. The 
draftsmen soon become so expert at this that a piece i| inches 



GENERAL PROCEDURE 9 1 

square can be cut out and another pasted in, in less time than 
it could be erased from tracing cloth. The piece to be changed 
is first squared off and cut out with a knife. This is then laid 
over another piece which is made ■£$ or § inch larger than 
the piece that has been cut out and the edges of this new 
piece are glued all around with ordinary library paste; it is 
then pasted on the reverse side of the drawing. The blue- 
print will be rather light around the edges of the patch, but 
this only indicates to the shop man where the changes have 
been made, and is rather an advantage than otherwise. 

Another method of making changes on bond paper draw- 
ings is as follows: When drawings have to be patched, a sheet 
of clean paper is laid under the part to be changed, which is 
removed by cutting with a sharp knife. The knife passes 
through both sheets of paper, thus providing a patch to fill 
the opening at the same time. To fasten this to the main 
body of the drawing, a piece of transparent paper spread with 
clear mucilage is used, if the patch is small. If of consider- 
able size, the joint is neatly covered with thin strips of 
gummed transparent paper about f inch wide. The advan- 
tage of this method is the smoothness of surface produced. 
The patch is flush with the main body of the drawing paper 
and the drawing instruments pass over the joint between the 
old and new portions without difficulty. It would be especially 
useful in cases where alterations are made on thick drawing 
paper. When neatly done with a sharp knife, the joint in 
such cases is almost invisible. 

Special Drawings for Patternmakers. — It is the practice 
of most shops to make a working drawing of a casting, which 
suffices for both the patternmaker and the machinist. The 
patternmaker gets a blueprint and often from a maze of lines 
picks out those which pertain to the pattern. The pattern- 
maker usually decides the amount of stock necessary for 
finish and, if the drawing is rather complex, he often loses 
time in distinguishing pattern dimensions from those neces- 
sary for machining operations. Drawings intended for use 
in both pattern shop and machine shop may also confuse the 



92 MECHANICAL DRAWING 



\ 



machinists who work repeatedly on this piece, and who are 
compelled to use a drawing bearing a lot of pattern dimen- 
sions. Very often, too, these drawings are made at a reduced 
scale. This necessitates various radii to insure the correct 
form for curved surfaces. 

At the works of the Fellows Gear Shaper Company special 
drawings are made for the patternmaker and this practice 
has been adopted by other manufacturers. These special 
drawings are full size and the amount of finish is decided in 
the drafting-room. No blueprint is furnished, but a buff 
paper drawing is made, and when the pattern is completed, 
the drawing is indexed and placed on file. This method has 
the following advantages: The full-size drawing gives the 
draftsman a better idea of proportions. By making the 
pattern drawing first and using this together with the assem- 
bly drawing when making details for the shop, the chance of 
error is much reduced. This is because a pattern drawing 
must be more thoroughly developed than a shop drawing. 
The making of several sections and an elevation or two not 
necessary to the machinist, often brings to light interferences 
which would otherwise escape notice. 

It might be said that the other method of making com- 
bined drawings would accomplish this. It is, however, im- 
practicable to make such pattern drawings as complete as 
they should be. It is the lot of the patternmaker to form 
many irregular curves. When the drawing is full size, he 
may prick through the paper and define a line on a thin board 
which, when cut to the fine, is a correct templet for the curve. 

Patternmakers Blueprints. — It is sometimes customary to 
make two tracings in order to secure separate drawings for the 
pattern shop and machine shop, but the following method 
has the advantage of requiring only one tracing. A finished 
tracing is made containing all dimensions both for the pattern- 
maker and machinist. The dimensions for the machinist are 
inked in as usual, but the pattern dimensions are put in with 
a soft lead pencil. Several prints are taken from the tracing 
while in this condition, one furnished the pattern shop, and 



GENERAL PROCEDURE 93 

as many filed away as desired. The lead pencil dimensions 
are then erased and the tracing is ready for making prints 
for the machine shop. In this way the patternmaker can 
readily understand and pick out his figures, and the machine 
shop print is kept free from unimportant dimensions which 
often cause considerable trouble. 

Commercial Side of the Draftsman's Work. — Machine 
designers should aim to develop a knowledge of commercial 
conditions, to acquire the business man's point of view in 
working out designs of machines. It has been well stated that 
an engineer is a man who not only designs and builds safely, 
but who also produces a machine that is adapted to manu- 
facturing needs at a cost commensurate with the service the 
machine is to render. One of the best known machine tool 
designers in the United States, who later became an executive 
of one of our largest machine tool plants, said that the great 
failing of most machine designers was their lack of apprecia- 
tion of the element of cost. "Anybody," he said, "can de- 
sign a machine to do almost anything, if he can spend all the 
money necessary; but it takes a real designer to so design a 
machine that it can be built and sold at a profit." 

The most successful machine designers are those having 
the idea of cost uppermost in their minds, so that while work- 
ing out the design they constantly take into consideration the 
commercial requirements referred to. The competent designer 
aims to originate mechanical devices that are reduced to their 
simplest form, because he knows that every unnecessary 
screw, pin, wheel, or lever increases the first cost, and possibly 
the upkeep. He also considers carefully the manufacturing 
problems, such as are encountered in the pattern shop, ma- 
chine shop, or foundry. A machine may be simple in its 
arrangement, perfect in its mechanical action, and have every 
part proportioned to resist safely all working stresses, but 
still be greatly lacking in design. The drawing of lines on 
paper is so much easier than forming in iron or steel the parts 
that the lines represent that the inexperienced designer does 
not always see his drawing from the manufacturing point of 



94 MECHANICAL DRAWING 

view. The cost of originating a design that may be drawn 
on paper in a few hours or days is usually insignificant when 
compared with the actual manufacturing costs which follow 
and often extend over a period of years. If a ioo per cent 
increase in the designing cost will reduce the manufacturing 
cost even i per cent, or less, this additional expense may 
yield a very high return. 

Value of Cooperation between Designing and Manufactur- 
ing Departments. — The value of cooperation between the 
designing and manufacturing departments is not always fully 
recognized, especially by the young engineer. What the 
shop men think of a new design is usually worth knowing. 
Frequently, a shop foreman or the men under him are able to 
suggest changes that simplify their work without reducing 
the effectiveness of the design as a whole. While engineering 
training and ingenuity are essential in the drafting-room, a 
machine designer is successful in proportion to his ability to 
simplify both the design and the methods of production. 
The best designer is one who aims for both mechanical and 
commercial success in whatever he originates or develops. 

A good draftsman and designer should be acquainted with 
the work of the patternmaker, molder, blacksmith and machin- 
ist. He should have a general knowledge of the principles of 
patternmaking so that his designs will not involve useless 
expense in the making of patterns; of molding, so that he will 
not be designing parts almost impossible to mold or on which 
there would be time wasted, unnecessary expense in coring 
out, or other operations which might be avoided if the design 
were made to conform to good practice in the foundry; of 
blacksmithing, so that the forging operations will not be un- 
necessarily difficult or expensive; of the machinist's work, so 
that the parts will be easily machined and all possible hand 
work avoided. The draftsman should also be competent to 
give the dimensions in such a manner that they will be the 
ones needed by the machinist in getting out his part of the 
work. He should plan in his mind how the parts will go 
together, starting from the foundation and following through 



GENERAL PROCEDURE 95 

the assembling of all of the parts till the machine is com- 
pleted. In short, the draftsman must be able to follow the 
work intelligently throughout the whole factory. 

A knowledge of machine shop practice is of especial import- 
ance. The draftsman should carefully consider how every 
casting or forging should be machined to enable the work to 
be done efficiently. While designers naturally think of finished 
surfaces, bearings, drilled and tapped holes, recesses, etc., 
there are many who do not consider the matter from the 
viewpoint of economy. 

The analysis of manufacturing problems as they come up 
in design is something like this: Can the part be machined 
readily? How? On what type of machine? Can the various 
members be assembled without difficulty? When it is decided 
that the work can be machined on some of the standard tools, 
many designers do not consider it necessary to go into the 
subject further, unless they have had practical experience in 
tool design and manufacturing. There are many important 
points in connection with machine designing that materially 
affect the cost of manufacture, such as the location of holes 
that must be drilled and tapped, general shape of the casting 
or forging, whether projecting lugs should be a part of the cast- 
ing or be attached to it, use of temporary flanges or lugs for 
holding or driving, the coring of internal grooves or recesses 
to avoid machining, etc. Of course, the effectiveness of a 
piece of mechanism must be the first consideration, and the 
weight, strength, and even general appearance are important 
points that must be considered; in fact, the methods of manu- 
facturing may, in some instances, be secondary, but it is essen- 
tial to study them carefully. 

Improving a Design after Mechanism is Constructed. — A 
drawing which is to furnish directions regarding the manu- 
facture of a part of some standard machine that has been 
thoroughly tested and has reached the manufacturing stage, 
should contain specific information regarding the size, finish, 
material, etc. 

In the development of a new mechanism on the drawing- 



96 MECHANICAL DRAWING 

board, however, there is a definite limit to the ideas that can 
be laid out in the drawing. After a design has been carried 
so far on a drawing-board, it is often essential that a model 
be made. The making of this model will no doubt bring out 
ideas that will make it advisable to redesign the mechanism 
on the drawing-board. Thus it is essential that the closest 
cooperation exist between the makers of the experimental 
mechanism and the engineering department. Also, no pains 
should be spared to acknowledge the value of an idea sub- 
mitted by a man in the shop who may know little about ma- 
chine design, but nevertheless be able to suggest changes that 
will improve the design. If, in a mechanic's opinion, some 
improvements over the method shown on the drawing can be 
effected, his ideas should be considered. There are instances 
where four, five, or six machines are built before one is fully 
satisfactory, but each one develops improvements. A drawing 
was required to start the first one, so the real purpose of the 
drawing was to make the start. When the machine is satis- 
factory, the drawings are corrected for record and duplication. 
It is important to remember that the drawing is a means to an 
end and not the end sought — a point sometimes lost sight of 
by draftsmen and by technical students. 

Property Rights in Engineering Drawings and Data. — The 
relations of the draftsman and engineer as regards his pro- 
prietary right in the designs which he creates, is closely allied 
with that of ownership of patents which originate during the 
work carried out by the draftsman. The subject may be 
divided into four principal questions, as follows: 

1. May a draftsman make blueprints from his own draw- 
ings embodying his own computations, and take these home 
with him or away with him when he leaves his job? May he 
do this with the drawings of his fellow workers? These will 
enhance his value to any subsequent employer. Are they his? 

2. Suppose he bought his own paper, and did his printing 
at home on Sundays and holidays, so that his records were not 
made at his employer's expense? Does this change any- 
thing? 



GENERAL PROCEDURE 97 

3. Suppose that this same information, tables of sizes and 
proportions, design data and standards, are in note-books. 
May the draftsmen copy these, and carry such priceless infor- 
mation gathered through years of wage-paying and experi- 
ment with him to his next place, and perhaps to a competitive 
concern? 

4. Can an improvement in a process, or a new process or 
an improved design, or a new mechanical movement be 
patented by the draftsman for himself, while he is working for 
an employer on a similar problem, and be used to hold up 
his employer until the parties can agree as to the terms? 

The accepted answer, emphasized by decisions of Court, 
and embodied in codes and standards of professional ethics, is 
that drawings and data belong to the employer, and the engi- 
neer or draftsman may not take them away with him. The 
reasons back of this practice include: (1) The shop furnished 
the plant — rent, heat, light, tools, etc. — where these ideas 
were conceived. (2) The shop presented the problem — with- 
out this the invention or the design would never have been 
created. (3) The shop furnished antecedent knowledge and 
acquired experience, which molded the creation, and pre- 
vented mistakes and waste. (4) The shop furnished experi- 
mentation, actual or precedent, which gave the creation its 
practical or commercial shape. (5) For many creations, the 
shop furnished or will furnish the manufacturing facilities 
which the inventor would otherwise have to struggle to find 
or pay for heavily elsewhere. (6) For many creations, the 
shop furnishes the selling or marketing facilities of its com- 
mercial organization. 

Again, the draftsman or engineer may contract by a signed 
instrument to give shop or manufacturing rights to the em- 
ploying shop, while retaining the right to sell or license to 
outside parties. Or, again, this principle may be made ap- 
plicable to patents which relate to the employer's business, 
while patents in no way related thereto may be expressly 
excluded, and the employer stands as an outsider would in 
relation to purchase or license. 



CHAPTER V 
SECTIONAL VIEWS AND THE READING OF DRAWINGS 

An object may sometimes be more clearly represented on 
a drawing by showing the cross-sectional shape. This is done 
by imagining the object to be cut as though it were literally 
cut apart by means of a saw; a drawing is then made which 
represents the exposed edges and surfaces. An outline draw- 
ing of a globe valve is shown at the left in Fig. i, and a sec- 
tional view to the right, which shows the valve as it would 
appear if split vertically through the center. The interior 
parts might be represented by means of dotted lines, but in 
this case, as in many others, a sectional view is much clearer. 
(The dimensions are omitted on these drawings to avoid a 
confusing mass of lines.) All parts cut by this section plane 
or cutting plane are shown by parallel " section lines" drawn 
at an angle of 45 degrees. As a rule, these lines are about 
Ye inch apart, but they are usually spaced entirely by the eye; 
the general tendency is to space the lines too closely together 
at first, and then to increase the space between them as the 
lengths of the lines increase. 

The arms of pulleys and gears are not section-lined, and if 
the cutting plane passes through the axis of a solid part like 
a shaft or bolt, which is one of the details of whatever part 
the drawing represents, ordinarily the shaft or bolt is not 
sectioned, thus making the drawing easier to read. Sections 
are usually made parallel to, or coinciding with, the long axis 
or length of the object, or at right angles or oblique thereto. 
They are then known as longitudinal sections, right or cross- 
sections, and oblique sections, respectively. When an object is 
supposed to be cut into two similar parts (as in Fig. 1), the 
view obtained by looking in a direction at right angles to 
the cut surface is called a half section. A view that shows 

98 



SECTIONAL VIEWS 



99 



the object cut in to the center on two planes at right angles 
to each other is called a quarter section. 

The cutting plane may be assumed to lie at any angle neces- 
sary to bring out the details most clearly; or a sectional view 
may represent an object as though it were cut through a part 
of the distance on one plane, and the rest of the way on an- 
other plane, either higher or lower, as may be required. All 



-G 



3- 






VERTICAL SECTION 



Machinery 



Fig. 1. Outline Drawing and Sectional View of a Globe Valve 



that is necessary to have the view clearly understood is to 
draw a line through one of the views of the piece, indicating 
just where the sectional view is supposed to be taken, and 
then to make a note on the drawing to that effect. 

Examples of Sectional Views. — In Fig. 2 is shown a side 
view of a handwheel and two kinds of sectional views. As 
the wheel is symmetrical, it is quite unnecessary to draw more 
than half the wheel in the side view, although the whole wheel 
may be drawn if desired. It is here represented as though 



IOO 



MECHANICAL DRAWING 



cut in two along its diameter on the line ab. This line should 
be a dash-and-dot line, as shown, and not a solid one. One 
of the uses of a dash-and-dot line is as a center line where a 
piece is symmetrical, and its use here would indicate that the 
half of the wheel not drawn was like the part that was drawn, 
even if it were not otherwise apparent. 

Above the center line cd, of one sectional view, the shapes 
of the rim and hub are shown by dotted lines, since they would 




r~\ 




f~\ 












Wr^^-^vv^; 



^ 

^ 



,^ 



<\> 






Machinery 



Fig. 2. Different Methods of indicating the Shape of a Section 

not be visible to an observer who held the wheel so that he 
looked directly at the edge or rim. Below cd is a sectional 
view taken along the line ab of the side view. 

To the extreme right, are shown two methods of drawing 
what are termed "dotted sections." The sections are sup- 
posed to be taken on the line ab as before, but cross-sectioning 
is done by dotted lines, indicating that the shape of the section 
would be as shown, but that the parts in front of it have not 
actually been cut away. This is a very convenient method 
to adopt at times. For example, in showing a milling ma- 



SECTIONAL VIEWS 



IOI 



chine knee and saddle it would enable one to represent the 
knee and saddle as they actually appeared, and also to show 
a sectional view of the mechanism under the saddle and inside 
the knee. If, on the other hand, the view were drawn as 
though the knee were actually cut through, one would not 
form an idea of its exterior appearance unless another view 
were drawn. It wall be noted in the figure that the dotted 
lines extend clear across the section, as drawn below ef, and 
only along the edge of the section above ef. 




t7 



{Mfrhy 



i wzd ' 



SECTION AT A B 



Machinery 



Fig. 3. Conventional Method of showing a Section of a Spur-gear Wheel 

In Fig. 3 are two views of a gear wheel. The one at the 
left side is a side view, and to show the shape to which the 
arms are to be formed, a sectional view of one of the arms is 
drawn in this view. The end of the shaft is supposed to be 
broken off and is sectioned. 

The right-hand sectional view is taken along the line AB. 
It will be noted that the shaft and key are not sectioned. 
The method followed in such cases is usually to section the 
castings or inclosing parts, such, for example, as the hubs, 
rims, etc., of a wheel, but not inclosed parts like shafts, rods, 
bolts, keys, etc. A bushing being both an inclosed and inclos- 



102 



MECHANICAL DRAWING 



ing part might or might not be sectioned, individual judgment 
dictating the method here as elsewhere. This gear has five 
arms, and the line AB cuts through one of them only. They 
are not sectioned in the right-hand view, and two opposite 
arms are drawn as though both of them lay in the plane of 
the paper. While this is not correct, it is the method usually 





Machinery 



Fig. 4. (A and B) Two Methods of sectioning Parts bolted together. (C) Section 
of a Casting having Three Lugs 



followed. The method of representing the gear teeth in sec- 
tional views is generally as shown. (The common methods of 
drawing gears will be dealt with in Chapter IX.) 

Sectional and top views of a cylinder or pipe on which a 
blank flange is bolted are shown at A and B in Fig. 4. There 
are five bolts, and the plane in which the sections are taken 
would cut through only one of them. Most draftsmen, how- 



SECTIONAL VIEWS 



103 



ever, would draw the sectional view as indicated at A. The 
bolts are shown as though both were in the plane of the sec- 
tion, and these bolts are not sectioned, but are drawn in full. 
It is not necessary to show more than two of the bolts, since 
it would detract from the clearness, and the top view shows 
plainly how many bolts there are. Some draftsmen think 
bolts drawn in this way are too prominent, and prefer to rep- 
resent them in sectional views as shown at B. This method 
also has the sanction of fairly common usage. When two pieces 
of the same material join, the section lines incline in opposite 
directions, as shown at A and B. 

Sketch C, Fig. 4, is another example of a figure that is not 
symmetrical in all respects. It shows two views of a step 
bearing having three ears or 
lugs for bolting it to its base- 
plate. In making a sec- 
tional view of such a piece 
the cutting plane is sup- 
posed to pass through the 
lugs in most cases, and, ac- 
cording to common practice, 
the sectional view would be 
made symmetrical, and the 
distance x in the lower 
view, from the center of 

the piece to the outer end of each lug, would be made equal 
to the distance x in the upper view. 

Section Lines. — When an object is cut by a section plane, 
the section lines show whether it consists of one or more 
pieces. Because the lines are uniform and run in the same 
direction at C, in Fig. 4, it is known that this part consists 
of only one piece. The change in the direction of the lines 
in Fig. 5, which represents a sectional view of a shaft bearing, 
shows that the shaft is located between two blocks placed in 
a casting to which a top piece is bolted. The two vertical 
dotted lines show that the bearing blocks have flanges while 
the horizontal white space in the center shows that the blocks 




Fig. 5. Use of Section Lines to indicate 
Different Kinds of Metal 



7L 



io4 



MECHANICAL DRAWING 



do not quite fill the space and so do not bear upon each 
other. 

When more than two parts are located close to one another, 
the section lines of some may be drawn at an angle of 30 or 
60 degrees with a horizontal rather than at 45 degrees, so 
that the outlines of each part may be more readily distin- 
guished, but this is not common practice. When the sec- 
tioned parts are very narrow and not easily sectioned, they 
are often shown as solid black sections. 






Cast Iron. 



Brass or Composition. Rubber or Vulcanite. 






Wrought Iron. 



Copper. 



Leather. 






Steel. 



Lead or Babbitt. 



Wood. 



Fig. 6. Standard Cross-section Lines adopted by the United States Navy 
Department 

Indicating Different Materials by Section Lines. — If the 

parts cut by the section plane are of different materials, the 
fact is usually shown by changing the arrangement or style 
of the section lines. In Fig. 5 the section lines show that 
the shaft is made of one material, that the two blocks enclos- 
ing it are made of another, and that the inclosing casting and 
top piece are of a third material. In Fig. 6 are shown the 
standard sections adopted by the United States Navy Depart- 
ment. Similar conventional methods have been adopted in 
different large drafting-rooms, but there is no universal stand- 
ard for section lines. The sections shown in Fig. 6, however, 
are in very common use. Some draftsmen section all parts 



SECTIONAL VIEWS 



I05 



alike, using the form that in Fig. 6 designates cast iron, and 
then printing on each part the name of the material of which 
it is to be composed or placing upon it a number. When a 
number is used to designate a material, it can also show the 





SOLID CYLINDER 



HOLLOW CYLINOER 



RECTANGULAR BAR 



<— — J 



£=1_=3 



r 



___ 



s 




Machinery 



Fig. 7. Broken Sections and Structural Shapes 

physical and chemical properties, such as the quality, heat- 
treatment, composition, etc. It also has the advantage of 
being easily changed should the material prove unsatisfactory, 
which cannot be done in the case of section lines. 

Broken Sections. — In Fig. 7 are shown methods of repre- 
senting bars and rods, shafting, structural beams, etc., when 




Fig. 8. Three Methods of representing Sectional Views of Bracket in Plane xx 

it is not convenient to show their whole length on the drawing. 
These pieces may be drawn as long as the limits of the draw- 
ing will allow, and then are broken as indicated, to show that 
the full length of the piece is not represented. The nature of 
the break indicates the cross-section of the piece. Drawing 
a broken section often permits using a larger scale. When 



io6 



MECHANICAL DRAWING 



placing the dimensions on the drawing, the full length is, of 
course, given. When drawing I-beams, angles, channels, etc., 
either the approximate shape of the section or the accurate 
shape is shown on the end. 

Sectional Views of Ribbed Parts. — The purpose of any- 
drawing is to be useful, and to this end care should be exer- 
cised in the use of conventional or common methods of repre- 
senting parts in drawings so that they will not be misleading. 
Just as orthographic projection cannot be strictly adhered to 
in all cases, so conventions are not universally applicable. 
Perhaps in no instances do more conventions appear than in 
the representing of ribs and in the sectioning of symmetrical 




Fig. 9. Two Methods of showing Section of Casting having a 
Central Rib 



and ribbed constructions. A number of cases are illustrated 
and comments are made on the various methods of represen- 
tation. These are only types, and are not intended to cover 
every possible case, but simply to illustrate possible correct 
solutions of typical cases. 

Sectional views on planes passing through ribs will be con- 
sidered first. Figure 8 shows a bracket with a section taken 
on the line X-X. The true section is shown at A, the dotted 
lines indicating the flanges, but it is hardly necessary to say 
that no draftsman would think of using such a representa- 
tion. A generally accepted method is shown at B, in which 
the plane of the section is assumed to have been moved for- 
ward until it is in front of the rib. Another method which is 



SECTIONAL VIEWS 



107 



sometimes used is shown at C, where alternate section lines 
are omitted from the rib. It will also be observed that in 
this case the flanges are shown by dotted lines instead of full 
lines, as the section is taken through the rib. The wider spac- 
ing indicates the presence of a rib of small thickness. 

Figure 9 represents an object having a rib as shown in the 
view at the right. The section on X-X as ordinarily repre- 
sented is shown at A, where it will be observed that the pres- 
ence of the rib is not so evident as it was at B, Fig. 8. The 
view A, Fig. 9, would 
be the same if there was 
no rib. It is sometimes 
necessary to show such 
an object in one view, 
and it is for such cases 
that the treatment indi- 
cated at B is found de-* 
sirable. One view with 
such a representation 
would clearly indicate 
the fact that there was 
a rib. However, this 
treatment of a rib is 
often convenient and 
desirable in other cases, Flg * 10 * 
as shown in Fig. 10. 
Note that the outline of the ribs where double sections are 
used is shown by dotted lines where they join the rest of the 
object. 

Figures n and 12 show ribbed flanges. At A, Fig. n, is 
shown the true section on X-X, which some draftsmen use, 
but the representation at B is more generally acceptable, and 
would be considered the correct one to use. The alternate 
method shown at C is also a correct representation, but is 
hardly necessary for such cases. At A, Fig. 12, is shown a 
cross-section on X-X which is projected, but it is not as evi- 
dent as the representation shown at B which would generally 




Sectional View of Plate having Ribs 
and a Central Hub 



io8 



MECHANICAL DRAWING 



be considered the correct one. The cross-sections at B or C 
might be used without the top views by simply giving the 
number of ribs in a note, and this would not be likely to lead 
to any misunderstanding. The representation at A would 
not be as clear. The vertical projection lines in Fig. 12 serve 
to show the method of obtaining the views A and B. The 




_ — __ — — , , , . 



h 







Machinery 



Figs. 11 and 12. Methods of cross-sectioning Ribbed Flanges— 
Methods B and C Represent Good Practice 

objection to the method shown at A is that the ribs do not 
appear in their true length. The objection to the representa- 
tion at A j Fig. 11, is that it indicates undue solidity. 

Sectional Views which are Symmetrical. — The subject of 
symmetrical parts, as far as sectioning is concerned, is closely 
related to the drawing of ribbed parts in section. Figure 13 
shows a section on X-X at A, in which the lugs are projected. 



SECTIONAL VIEWS 



109 



This gives the idea of lack of symmetry. The lug lying on 
X-X is treated as a rib and so is not sectioned. At B is 
shown a more desirable representation which should generally 
be used. Figure 14 shows the treatment of a similar object, 
A being the true section on X-X and B the preferable 




Figs. 13 and 14. Two Kinds of Cross-sectional Views 

method of representation. Figures 15 and 16 show the treat- 
ment of circular objects with arms, A being the incorrect and 
B the correct representation in each case. The alternate 
method shown at C is another correct representation of the 
object shown in Fig. 15. The projection lines show the 
method of obtaining the sections. 

A similar condition arises in representing the holes in 
flanges, and C in Fig. 17 shows the correct method in such 



no 



MECHANICAL DRAWING 




Fig. 15. Other Examples of Symmetrical Cross-sectioning 

cases, where the distance between the holes is made equal to 
the diameter of the circle of drilling regardless of projection. 
At A is shown the true projection, which is misleading. At B 
the holes are shown dotted, the centers of the holes being the 



<&» 




Machinery 



Figs. 16 and 17. Cross-sections of a Handwheel — Sections of a Flange having 
Equally Spaced Holes 



SECTIONAL VIEWS 



III 



true distance apart. 
This is a correct repre- 
sentation and is often 
used when none of the 
holes are on the plane 
of the section. Key- 
ways and pins will be 
considered in the same 
manner. In Fig. 18, A 
shows the projected sec- 
tion of the hub with the 
keyway, while B is the 
representation often 
used to preserve sym- 
metry. At A, Fig. 19, 
is shown the projection 

of a hole through a shaft for a pin, and B and C are accept- 
able representations of this piece, even though the end view 
remains the same. 

The object in all cases is to give a representation which will 
most clearly convey the idea. To this end symmetrical 
objects should appear symmetrical, and the true size should 




Fig. 18. Methods of showing Keyways in 
Cross-sectional Views 




Fig. 19. Methods of showing Transverse Holes in a Shaft 



be shown in preference to a foreshortened one. Holes should 
appear round, and diameters should appear in their true length. 
Showing Part of Drawing in Section. — The general shape 
of a piece is often brought out more clearly by showing half 
of a drawing in section. A simple example is illustrated "in 
Fig. 20 which represents the rear wheel hub of an automobile. 



112 



MECHANICAL DRAWING 



As will be seen, the sectional view extends down only to the 
center line. This part section shows very distinctly the shape 
of the hub; the exterior view below the center line shows the 
threaded end to better advantage than would a complete 
sectional view. While a complete section could be used to 
represent a simple part of this kind, in some cases an exterior 
view of one half of a piece, which also requires a sectional 
view, is very essential. 




REAR WHEEL HUB - STEEL CASTING 



Machinery 



Fig. 20. Automobile Wheel Hub which is shown partly in Section 

Another example of a part section is shown in Fig. 21, 
which illustrates a conical shaped " cinder pot" having flanges 
at each end and a bottom plate bolted to the lower flange. 
The sectional view to the left of the center line shows dis- 
tinctly the shape of the flange which, as will be noted, is not 
so clearly revealed by the exterior view at the right. On the 
other hand, the right-hand half of the drawing shows the 
stiffening ribs better than the sectional view. A working 
drawing of this pot might have a plan view showing the 
number of ribs and bolts, but frequently such information, 
especially in the case of simple drawings, is given by notes. 
For instance, this pot might have the number of bolts and 
ribs marked on it in order to avoid a plan view. 



SECTIONAL VIEWS 



113 



Sectional Views of Important Details. — There are no gen- 
eral rules governing the sectioning of drawings except the 
rule that sections should be used wherever they make the 
drawing a clearer and better representation of the object 
drawn. In many instances, sectional views of important de- 
tails are shown, the idea being to show certain essential parts, 
just as sections are cut out of some models used for demon- 
strating purposes, to show the interior arrangement. In this 
way, the drawing is made to bring out distinctly some part 




Machinery 



Fig. 21. Another Example of a Drawing shown partly in Section 

of a mechanism which would be rather obscure if not shown 
in section. One example is illustrated in Fig. 22. This view 
shows the mechanism for revolving or indexing the turret on a 
Gridley single-spindle automatic. In this case, part of the 
worm and worm-wheel is shown in section, and that part of 
the machine containing the plunger which engages the notched 
indexing-disk or wheel. The sectional view of the worm 
shows the form and arrangement of the details much better 
than would have been possible without a section. The ad- 
vantage of a sectional view through the index-plunger housing 
is also apparent. These part sections are very commonly 
employed in mechanical drawings, and the draftsman must be 



U4 



MECHANICAL DRAWING 




SECTIONAL VIEWS 1 15 

guided somewhat by judgment in determining when sectional 
views are preferable. 

Indicating Position of Part Shown in Section. — The rela- 
tion between a sectional view and some other view such as a 
plan or elevation, is not always apparent unless the location 
of the section is indicated in some way. Furthermore, two 
or more sectional views of the same part are often required. 
It is evident that the sectional view of the globe valve (Fig. 1) 
shows the valve as it would appear if cut in halves vertically 
through the center, but the relation between the different 
views on drawings which are a little more complicated is 
not always as apparent, and the relation between the section 
and other parts of a drawing is frequently shown by mark- 
ing on one view the section or cutting plane represented by 
another sectional view, which is marked in a similar manner 
to identify it. 

Figures 23 and 24 show different views of a steering gear 
case. (While these views are shown on two separate illustra- 
tions in order to reproduce them on a larger scale, they all 
belong to one working drawing.) Upon reference to Fig. 23, 
it will be noted that the sectional view is marked " Section 
A- AT By referring to the plan view, the location of the 
cutting plane A- A may be seen, as this plane is represented 
by the line ending with arrows and marked with a letter A 
at each end. The arrows show which side of the sectioned 
part is represented by the sectional view. These arrows are 
not always used, but they often tend to facilitate reading a 
drawing. The sectional view just referred to is taken in a 
vertical plane, whereas the one shown in Fig. 24 represents a 
view in a horizontal plane. This latter section is marked 
" Section B-B," and by referring to the elevation, or side view, 
beneath it, the line B-B shows that the section represents the 
casting as it would appear if cut through the center horizontally 
along this plane. 

Section Representing more than one Cutting Plane. — 
Most sectional views represent the part as though it were 
cut straight through on one plane, which may or may not 



u6 



MECHANICAL DRAWING 



^4^ 



0-505 t°o.Z REAM 




SECTION A-A 



Machinery 



Fig. 23. Automobile Steering Gear Case — Note how Relation of Sectional View 
to Plan View is indicated 



SECTIONAL VIEWS 



117 



r^ ^f^ ^ 




FT 



3-? 



Machinery 



Fig. 24. Additional Views of Automobile Steering Gear Case illustrating how 
Position of Sectional View is indicated 



n8 



MECHANICAL DRAWING 



coincide with the center line; but sometimes the cutting plane 
is assumed to pass through the part to a certain point and 
then change its direction. An example is shown in Fig. 25. 
The view at the left, marked "Section A- A" represents the 
section along the cutting plane A- A as indicated on the right- 
hand view. As will be seen, this cutting plane extends ver- 
tically down to the center of the circular part of the casting 




Machinery 



Fig. 25. Sectional View representing more than One Cutting Plane 

and then continues at an angle through the extension pro- 
jecting from the lower left-hand side. The advantage of 
making the drawing this way is that the important features 
are clearly shown in one sectional view. For instance, the 
lugs at C and D are shown, and then by drawing in section 
the projecting part, the general shape of the entire casting is 
brought out much better than by merely representing the 
section as it would appear if the cutting plane passed straight 
through the casting either vertically or at an angle. While 
the sectional view to the left is not a true drawing according 



SECTIONAL VIEWS 



119 



to the projection method of making mechanical drawings, it 
does show clearly the shape of the casting, which is the essen- 
tial requirement. A separate detail section, marked on the 
drawing " Section B-B" is included to show the cross-sectional 
shape of the casting between the bolt lugs, the location of this 
detail section being represented by the line B-B on the right- 
hand view. 

If the cutting plane changes its direction, more than two 
letters may be used, to show clearly the different points at 




- — - 

. „' WW 

w,x^d 



^ 



Machinery 



Fig. 26. Another Method of denoting Sections on Drawings 



which changes occur. For instance, a sectional view such as 
the one shown in Fig. 25 might be marked "Section A-B-C" 
these three letters being placed on the right-hand view to 
show the path followed by the cutting plane. The letter B 
would, in this case, be placed at the point where the vertical 
cutting plane changes its direction. A letter at each end of 
the line representing the cutting plane usually shows clearly, 
however, the position of the cutting plane. 

Another method is to use one letter only for any particular 
section, every change in the direction of the cutting plane 
being denoted by the same letter, as illustrated by Fig. 26. 



8l 



120 MECHANICAL DRAWING 

The location of the lines and letters on the view to the left 
shows that the sectioned part (see right-hand view) represents 
sections in three different planes. Incidentally, this illustra- 
tion shows a feature of drafting practice not used by drafts- 
men as much as it could be advantageously; i.e., showing the 
cross-sections by means of dotted lines instead of full lines. 
This use of dotted lines enables the object to be drawn in full 
lines as though it were not illustrated in section, so that in 
the same view sections on any parallel plane may be repre- 
sented without interfering with the remainder of the drawing. 

Showing Sections without Use of Section Lines. — The 
evenly spaced section lines which are used on most drawings 
to indicate cross-sections, are not employed by some drafts- 
men, because this method of representing sections is considered 
tiresome and wasteful of time. Another method of showing 
cross-sections is as follows: A tinting ink is used made from 
ordinary black Higgins ink with sufficient alcohol added to 
thin it. A test may be applied to some white paper. When 
the ink dries a light brown, it is of the proper consistency. 
This ink or paint is applied to the reverse side of the tracing 
on the parts which are to be shown in section, using a small 
camel's hair brush. On the blueprint this sectional view 
shows a cloudy bluish white, which is very readily distin- 
guished. The main use of tinting ink is on all large full-sized 
sections, such as the beds of machines. The tinting ink is 
not applied all over the cross-section if the latter is large, as 
the cloth would be injured by the shrinkage that would result. 
A border of ink, about f inch in width, is painted around the 
outline of a large section. 

Another method, equally simple, which gives very similar 
results, is merely to rub over the parts shown in section on 
the back of the tracing with a soft pencil. On the blueprint 
this will also show as a bluish white. The pencil method has 
an advantage over the ink or paint method, as the latter has 
a tendency to wrinkle the tracing cloth if the thin ink is applied 
too liberally. 

While the ink or pencil methods of sectioning require little 



READING DRAWINGS 121 

time, regular cross-section lines are preferable for most draw- 
ings. The bluish-white color for representing sections on a 
blueprint is likely to cause trouble, for that same bluish white 
may be produced by a tracing which has become spotted 
with water — a thing which happens quite frequently where 
the sun exposure method of printing is used. 

Reading Mechanical Drawings. — The expression " reading 
a drawing" simply means obtaining a clear understanding of 
it, by referring to the different views. Experienced drafts- 
men, machinists, toolmakers, and patternmakers are all able 
to read drawings, although it does not follow that they could 
make a suitable drawing. Everyone can understand an ordi- 
nary perspective drawing, because it represents the object as 
it would actually appear to the eye, but a mechanical drawing 
with its different views, numerous full and dotted lines, dimen- 
sions, symbols or abbreviations is comparatively complex, 
although it shows to the trained eye a great deal more than 
would be possible, in most cases, by a perspective drawing. 
The first step in learning how to read drawings is to study 
elementary mechanical drawing principles. When the stu- 
dent understands the use of different views for representing 
different sides of a mechanical device and the use of other 
features common to mechanical drawings, such as dotted lines 
to represent concealed parts, sections, and the meaning of 
certain abbreviations, the ability to read drawings is soon 
acquired with practice. The best plan is to begin with simple 
drawings and then practice reading more complex ones, secur- 
ing as great a variety as possible. 

General Procedure when Reading Drawings. — When read- 
ing a drawing, it is advisable to visualize the object as far as 
possible, or see it in the mind's eye as it would appear when 
constructed. This is where the imagination comes into play 
somewhat and also the ability to grasp readily the relation 
between the different views by glancing from one view to the 
other. For instance, if there are front, plan, and side views, 
these separate views on the drawing are combined mentally 
so that the mental picture corresponds to that of the object 



122 



MECHANICAL DRAWING 



itself. Students of mechanical drawing are sometimes puzzled 
when attempting to read a drawing, because they expect every 
drawing to conform exactly to certain rules, and forget that 
a draftsman may not always make a drawing which is theo- 
retically correct, if, by some variation from the usual practice, 
he can represent the object more clearly. A section on one 
side of a center line may represent a different cutting plane 
than a section on the other side of the same center line, in 



| X 1 SET-SCREW 





^ 



C^ 



Machinery 



Fig. 27. Drawing of a Flange Collar 



order to show in one view the arrangement or shape of in- 
terior passages or other features. By carefully observing the 
lines on different views which represent the corresponding 
parts, such variations will be detected readily and understood. 
When the general shape or arrangement of the object shown 
on the drawing is clear, at least as far as the main features 
are concerned, the details and the dimensions should be ob- 
served. When studying the details, it is frequently necessary 
to glance from one view to another. For instance, it may be 
impossible to determine whether a circle on one view repre- 



READING DRAWINGS 



123 



sents a hole or a projecting boss of circular shape, until the 
corresponding lines on another view are observed. If the 
drawing is rather complex, there may be some doubt as to 
the relation between lines on different views, in which case a 
straightedge or T-square is sometimes used to project points 
from one view to another in order to determine definitely 
whether certain lines represent the same part. Dividers can 
also be used for this purpose, the method being to compare 




Fig. 28. Sectional and End Views of a Small Casting 

the distance between the lines on different views and a com- 
mon center line. 

Examples Illustrating how to Read Drawings. — A few con- 
crete examples will be considered to illustrate just how draw- 
ings are read and the points to be observed in understanding 
thoroughly what they represent. A very simple drawing is 
shown in Fig. 27 which represents a flanged collar. The 
side view shows clearly that this collar contains two set- 
screws located 90 degrees apart, and that the heads of these 
set-screws are within some form of pocket. The right-hand 
view shows clearly that this is a square pocket. Incidentally, 
the object of designing a collar in this way is to avoid accidents 



124 MECHANICAL DRAWING 

by so inclosing the set-screws that they cannot catch the cloth- 
ing, in case a workman should come into contact with the 
collar while the latter was attached to a revolving shaft. 

Another simple type of drawing is shown in Fig. 28. It is 
evident that the sectional view represents a section on the 
vertical center line. This sectional view shows that a lug 
projects into the central passageway and the right-hand view 
shows clearly the shape of this lug. The sectional view also 
shows that a ring having a beveled edge is inserted in a recess 
formed in the threaded end of the casting. The fact that this 
ring is a separate part is indicated by the section lines which 
incline in the opposite direction from the others. It will be 
noted that the section lines in both cases are the same, and 
according to the usual custom, these evenly spaced parallel 
section lines represent cast iron. It does not follow, however, 
that the inserted ring is of cast iron, because it is the practice 
in many drafting-rooms to specify by a note on the drawing 
what kind of material is to be used instead of relying upon 
different kinds of section lines which may, in some cases, be 
misunderstood, since there is no universal standard governing 
their use. On a working drawing of this part, the kind of fit 
between the ring and casting should also be indicated. If 
the ring is a press fit, as in this case, the allowance for the 
fitting should also preferably be given. 

The drawing reproduced in Fig. 29 represents the external 
brake-band anchor of an automobile. The left-hand view 
shows that there is an elongated hole in the casting, and the 
dotted lines of the right-hand view show that this hole extends 
through the casting. The dotted lines in these two views also 
show that there is a hole located at right angles to the one 
just referred to. The sectional view above shows very clearly 
these two openings, and also the shape of the casting in a 
plane A- A. This drawing illustrates the use of printed in- 
structions. When the patternmaker receives the drawing, he 
notes that the elongated hole, which is 1 inch long and if 
inch wide, is to be cored. Consequently, it is necessary 
to make, in addition to the pattern, a suitable core-box. The 



READING DRAWINGS 



125 



drawing also shows that the hole extending at right angles to 
the one to be cored is drilled out with a it-inch drill. The 




Machinery 



Fig. 29. Drawing of a Brake Band Anchor 

fact that the elongated hole is to be broached is also indi- 
cated, and the method of finishing the sides of the casting, 
which in this case are to be disk-ground. The radius of the 
main part of this casting is given as 7 inches, and the zig-zag 
radial line shows that the center of the arc is on the center 



126 



MECHANICAL DRAWING 



line of the casting. The use of these zig-zag lines for raoial 
dimensions is common, because, in many cases, if the radial 
line were extended out to the point where the center is actually 
located, it would be beyond the limits of the drawing. For 
this reason, the radial lines are frequently drawn zig-zag 
fashion merely to show from which center line the arc is struck. 
The drawing of a crane hook of 10 tons capacity is shown 
in Fig. 30. This drawing illustrates a simple method of show- 
ing cross-sectional shapes without using separate sectional 
views. In this case, the right-hand sectional view shows that 







di 


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Machinery 



Fig. 30. Simple Method of showing Cross-sectional Shapes by placing Sectional 
Views directly on Drawing 

the eye of the hook is of circular cross-section and the other 
sectioned part shows that the hook is somewhat V-shaped. 
The particular part of the hook which is given this V-shaped 
form is indicated by the line A, which represents the limits 
of the flattened surface. This method of placing cross-sec- 
tions right on another view and at the point where the section 
is taken, is often resorted to and is not only convenient, but 
frequently shows the shape of the object more clearly than 
would separate views. 

The drawing reproduced in Fig. 31 is more complicated 



READING DRAWINGS 



127 




SECTION A-A 

Machinery 



Fig. 31. Drawing of Cylinder Head Casting which illustrates Certain Points in 
the Reading of Drawings 

than those previously referred to, and illustrates several in- 
teresting features. As it was necessary to reduce this drawing 



128 MECHANICAL DRAWING 

greatly in order to show it on a book page, the numerous 
dimensions on the working drawing were omitted. The draw- 
ing represents the cylinder head casting of a gasoline engine. 
The different views, named in the order in which they appear 
and beginning at the top of the illustration, are as follows: 
A plan view, a sectional view, a bottom view, and another 
sectional view. A glance at these different views shows at 
once the general shape of the head. In making this drawing, 
the most important part of the draftsman's work was to show 
clearly the form and arrangement of the interior passages, 
which is done by the two sectional views in conjunction with 
the dotted lines on the bottom view. It will be noted, how- 
ever, that the sectional views are not confined to one cutting 
plane. The heavy dot-and-dash line B-B on the bottom 
view shows clearly the two planes represented by the section 
B-B above the bottom view. The fact that this section is not 
in one plane is indicated by the abrupt change in the form of 
the interior passages, which occurs at the center line. The 
right-hand half of section B-B shows that the cutting plane 
passes through a tapped hole a and by referring to line B-B 
on the bottom view the location of this hole is indicated. The 
shape of the opening b in the sectional view is also shown 
by referring to the bottom view. The section A- A repre- 
sents four different planes, as indicated by line A- A on the 
bottom view. The section first passes through the center of 
bolt hole c and is then shifted to coincide with the center of 
the circular space on the under side of the head. The cutting 
plane is again shifted to the right to a more central point, 
and then over to a plane which passes through opening d. 
By changing the cutting planes in this way, a sectional view 
A- A is obtained, which shows a great deal more about the 
shape of the casting than would a section representing one 
plane. 

In attempting to read this drawing the student will be 
assisted by referring to the small reference letters which repre- 
sent the same parts on the different views. A study of these 
views will show that the casting is of the same shape on each 



READING DRAWINGS 1 29 

side of the vertical center line. For instance, if the section 
B-B were taken in one plane, the sectional view would be the 
same on each side of the center lin*\ Therefore, instead of 
duplicating the view, each half of the section is taken in dif- 
ferent planes. As will be seen, the under side of the head at 
X is much lower than the under side at Y, which is in a dif- 
ferent plane, and by referring to the section A- A, the curva- 
ture of the head which causes this difference in height in sec- 
tion B-B is shown. Of course, on the working drawing, the 
radii of these curves and all other necessary dimensions are 
given together with whatever explanatory notes are needed. 

The few examples which have been referred to are intended 
to indicate in a general way the method of procedure when 
reading drawings. The student should practice reading the 
other drawings reproduced in this book and also drawings or 
blueprints obtained from as many different sources as possible. 



CHAPTER VI 
METHODS OF DIMENSIONING WORKING DRAWINGS 

As most mechanical drawings are used in the pattern shop, 
machine shop, or forge shop to show the workmen exactly 
what is required, these working drawings should contain all 
necessary dimensions and, in addition, whatever instructions 
may be needed to make every operation entirely clear. While 
it might be possible to measure an accurate drawing and in 
this way obtain the dimensions within fairly close limits, 
such a method, even when applied to simple parts not requir- 
ing great accuracy, would be very unsatisfactory, and often 
result in serious errors; but even if this method of obtaining 
dimensions by the direct measurement of drawings were 
practicable, it is much better to place all important dimen- 
sions on the drawing where they can be seen readily because, 
when a drawing is properly dimensioned, it shows exactly 
what is required. While it is usually desirable to make a 
drawing quite accurate, an inaccurate working drawing, 
properly dimensioned, is much superior, as a rule, to an accu- 
rate one which is not dimensioned or does not include all the 
dimensions needed in the shop. Even a rough free-hand 
sketch would ordinarily be better than a drawing made accu- 
rately but without adequate dimensions and instructions. 
In fact, a large percentage of the work done in machine shops 
and tool-rooms must be made so accurate that dimensions 
expressed in figures are absolutely necessary. The chief pur- 
pose of the drawing itself is to show to which parts or surfaces 
the given dimensions apply. 

When the dimensions are being placed on the drawing, the 
draftsman should keep in mind all of the different manufac- 
turing or machining operations that will be required, and 
place the dimensions on the drawing in such a manner that 

130 



DIMENSIONING WORKING DRAWINGS 131 

the workmen will be able to proceed without asking questions. 
A knowledge of shop practice is helpful to the draftsman in 
dimensioning drawings, as well as la other ways. 

General Methods of Dimensioning Drawings. — When 
dimensions are less than a certain amount (which varies in 
different drafting-rooms), the common practice is to express 
them in inches, but if the dimensions exceed this amount, 
they are usually given in feet or inches. For example, if 
twenty-four inches is the amount or the dividing line, twenty- 
two inches would be written 22", or simply as 22, without 
the double accent sign, which represents inches. If the di- 
mension were, say, three feet five inches, instead of writing 
41", it would be written thus: 3 ft. 5 ins.; 3 ft. 5"; or 3 '-5*. 
In one case, the word "feet" is abbreviated, and in the other, 
it is represented by a single accent mark. When the latter 
method is employed, a dash should be placed between the 
figures so they will not be mistaken for one number. For ex- 
ample 3 '5" might be read 35". If the dimension is equivalent 
to a whole number of feet, it is common practice to place a 
zero after the dimension to show unmistakably that it is cor- 
rect and that the inches were not omitted. Thus, 4 ft. o ins., 
or 4 / -o' / . If the dimension is four feet one half inch, it 
would be written: 4 ft. oj in., or 4'-o|". 

In some drafting-rooms, all dimensions of 36 inches or less 
are expressed in inches and in other plants this rule applies 
to all dimensions less than 48 inches. There are also some 
exceptions to this use of feet and inches. For example, the 
stroke of a steam engine is given in inches, regardless of its 
length, and the same is true of the length of the wheel-base 
of an automobile or locomotive. When all the figures on a 
drawing represent inches and not feet and inches, the figures 
are often given without double accent signs to indicate inches. 
When the dimension is either a whole number and a fraction 
or simply a fraction alone, a common fraction or a decimal 
fraction may be used. This point will be considered later. 

The dimension is ordinarily placed in a space left at the 
center of the dimension line, as at A, Fig. 1, but sometimes it 



I 3 2 



MECHANICAL DRAWING 



is placed outside of the extension lines, as illustrated at B, 
provided the space between the extension lines is small. The 
extension lines should not quite touch the lines of the drawing. 
Most dimension lines are either horizontal or vertical, and 
the dimensions for the vertical lines are usually placed so as 
to be read on the right-hand side of the drawing, as illustrated 
in Fig. 2. On some drawings, however, these vertical dimen- 
sions are placed so as to be read from the bottom of the sheet 
the same as the horizontal dimensions. It will be noted that 
the dimension lines are parallel to the side on which the di- 
mension is given. 

It will be understood that the dimensions on a drawing in- 
dicate the actual size, the scale of the drawing not being con- 




Fig. 1. Methods of using Dimension Lines 



sidered. The dimensions also indicate the finished size and, 
in the case of castings or forgings, allowance must be made 
in the pattern shop and forge shop for machining operations. 
Most patternmakers work from drawings which give the 
finished dimensions, and they allow on the pattern whatever 
is considered necessary for the machining operations. Special 
drawings for patternmakers, however, are sometimes furnished. 
Most dimensions and dimension lines are placed outside of 
the drawing proper where there is a clear space and where 
they are not so liable to be confused with the lines of the 
drawing itself. In a great many cases, however, the dimen- 
sions must be placed within the outline of the drawing. In 



DIMENSIONING WORKING DRAWINGS 



J 33 




134 MECHANICAL DRAWING 

fact, it is often preferable to have them right on the drawing 
and close to the dimensioned parts. When a dimension is 
placed on a sectional view, a clear space should be left for 
the dimension by omitting the section lines around it. One 
of the lines of the drawing should never be used in place of a 
dimension line, as this would lead to confusion. It is also 
bad practice to use a center line as a dimension line. 

The dimension line for the radius of an arc has an arrow 
at only one end (see sketch C, Fig. i) and the dimension is 
followed by the abbreviation "R." or "Rad.," indicating radius. 
When dimensioning circles, the diameter should be given in- 
stead of the radius, the latter being used only for arcs of circles. 
When indicating the distance between two circular parts, the 
dimension from center to center should always be given. 

If there are several dimensions in a row, the over-all dimen- 
sion should be included as a rule. This point will be con- 
sidered more fully later. The shorter dimensions are placed 
nearer the outline of the drawing or within the over-all dimen- 
sion line. When a piece has several different diameters 
throughout its length, as, for example, a shaft with shoulders 
of different sizes, the diameters should be given on the side 
rather than on the end view, because they show more clearly 
just which section any particular dimension applies to. This 
general rule also holds good for such parts as castings having 
holes or openings of different sizes. 

Dimensions should be placed on that view which shows 
most clearly the part of the drawing to which the dimension 
applies. Dimensions of the same part should not be repeated 
on different views because if changes have to be made some 
of the duplicate dimensions may be overlooked; moreover 
the number of lines on a drawing should be reduced as far as 
possible for the sake of clearness. The essential point is to 
give all dimensions that are necessary without useless repeti- 
tion. Another important point is to give dimensions which 
are related to surfaces from which a workman can and should 
measure directly, as this not only facilitates the work in the 
shop, but tends toward greater accuracy. 



DIMENSIONING WORKING DRAWINGS 135 

Standard Rules for Dimensioning Drawings. — The en- 
gineering departments of large manufacturing concerns have 
come to realize the necessity of adopting standard methods 
of dimensioning drawings. This necessity arises from the fre- 
quent mistakes made in reading dimensions on drawings 
when the draftsman is allowed to use any method that he 
prefers. It is only natural where many draftsmen are em- 
ployed that widely different methods will be used which con- 
fuse the foremen and machinists to whom the drawings are 
given. To avoid these mistakes, the engineering department 
of a large manufacturing concern furnishes each member of 
the engineering department with a copy of a data sheet giving 
the following directions (which are given as an example and 
not as a complete or perfect list applicable to all drafting- 
rooms) : 

All dimensions of 48 inches and under are to be shown in 
inches; dimensions over 48 inches are to be shown in feet 
and inches. 

The inch sign is not to be used upon gage drawings or upon 
tool equipment drawings, when the sizes are below 48 inches. 

Upon drawings for buildings, floor plans, etc., the foot and 
inch signs will be used, but will invariably be given the re- 
verse slant, in order to avoid confusion with the minute and 
second signs of subdivisions of the circle. 

The decimal subdivisions of the inch will be used invariably 
upon all drawings of machine and tool equipment; but the 
fractional subdivisions of the inch will be used upon drawings 
of buildings, floor plans, etc.; also in notes calling for stand- 
ard taps, reamers, bolts, etc. Decimal points should be 
made heavy. 

When a dimension is not to scale, it is to be underscored 
with a heavy black line. 

If more than two views of a piece are shown, the distances 
between the projected views are to be equal, care being taken 
that views are neither crowded nor too far apart. 

Unimportant dimensions, if any, should be left blank, so 
that workmen will know where to work close. 

QL 



136 MECHANICAL DRAWING 

Over-all dimensions should always be given. 

Arrow-heads should be made free-hand and about 60 de- 
grees between wings. 

The following general rules are given by the Chalmers 
Motor Co., and they show some of the minor variations in 
practice : 

All dimension lines should be placed outside of the views, 
as far as possible. Elevations and end views of an object to 
be dimensioned should be tied together with projection lines 
of the same weight as the dimension lines, with the dimen- 
sions placed between. 

All diameters of holes occurring in the body of the drawing 
should be carried out for dimensioning with full projection 
lines. Never place the diameter of a hole inside the hole 
itself, except when the hole is very large. Numerous diam- 
eters should never be marked transversely across the face, 
with the dimension lines intersecting a common center. 

All dimensions should read from the bottom and right-hand 
side, as far as possible. It is not necessary in motor car work 
to add the customary inch marks to the figures. Section 
through threads should be shown with two parallel lines ap- 
proximately the depth of the thread. The customary method 
of showing threads by inclined lines should be avoided. This 
has a tendency to blur a drawing badly. 

There should be an understood allowable variation or toler- 
ance in rough forgings and castings where a dimension is 
shown in a common fraction; on finished work where a com- 
mon fraction is given; and on finished work where a decimal 
is given. Very close limits or tolerances should be shown 
by writing the maximum and minimum dimensions allow- 
able, in decimals. 

Examples of Dimensioned Drawings. — In the dimension- 
ing of drawings, there are no inflexible rules which can be 
laid down, and it is necessary for the draftsman to exercise 
his judgment and experience. The method of dimensioning 
should be planned or thought out very carefully, just as the 
method of machining the work itself is planned. Figure 2 



DIMENSIONING WORKING DRAWINGS 137 

represents the working drawing of a cast-iron bearing box. 
This drawing illustrates some of the features of dimensioning 
which have been referred to,. As the sectional part shows, 
this casting has an annular recess in each end, and in each 
recess there is a shallow groove. This drawing illustrates the 
point regarding the placing of diameters on the side view 
instead of on the end view. In this case, the bore of the 
casting is marked on the end view, where it shows very clearly 
that the dimension (2I) applies to the inner diameter. While 
it would be possible to place the various other diameters on 
this end, the relation between them and the different surfaces 
is shown much more clearly on the side view. This drawing 
also illustrates the advantage of placing some of the dimen- 
sions within the outline of the drawing proper, as, for example, 
those representing the diameters of the bosses in the side view 
and the distances between these bosses. The four dimensions 
of 1 1 inches, if added together, give the total length of the cast- 
ing, but this total length is given at the top of the drawing so 
that the patternmaker or machinist will not have to do any 
adding. 

Another example illustrating methods of dimensioning is 
shown in Fig. 3, which illustrates a steel part of odd or irregu- 
lar shape. As the illustration shows, this piece has curved 
sections of different radii, and in dimensioning the drawing, 
it is necessary to give not only the radii but dimensions show- 
ing their centers. For instance, the center of the 4§-inch 
radius is located i^ inches from the horizontal center line and 
5-^2 inches from the vertical center line. The 2J- and 21- 
inch radii are each struck from a vertical line, which is it& 
inches to the left of the vertical center line. The 5|-inch radius 
shows that the length of the curved section at the left should 
be determined with reference to the T Vinch hole. When the 
center of an arc is not definitely located by dimensions, this 
shows that the location should be such that the arc joins 
neatly with the lines at each end. 

Drawings of Parts for which Tolerances are Specified. — In 
the manufacture of machine parts, perfection is impossible, 



^ 



MECHANICAL DRAWING 



although, with the machines and the precision measuring 
tools now available, it is practicable to finish work that is 
within exceedingly close limits of a given size. In ordinary 
manufacturing practice, however, such extreme refinement is 
unnecessary, and since it is also expensive, the modern method 




Machinery 



Fig. 3. Example illustrating Methods of indicating Radial Dimensions 

of constructing machinery and tools is to adopt certain toler- 
ances or allowable errors. The various parts forming the 
machine are then finished to some dimension which is within 
the minimum and maximum dimensions allowed. By the 
adoption of this method, parts can be produced which are 
accurate enough to serve all practical purposes, but not need- 



DIMENSIONING WORKING DRAWINGS 1 39 

lessly accurate, since the cost of production increases rapidly 
as the tolerance or allowable error is reduced. 

If a dimension is given on a drawing and nothing is said 
about the tolerance, the usual assumption is that the part 
does not need to be finished very accurately, because most 
drawings of castings, forgings, or other pieces which are not 
regarded as precision work usually have plain dimensions and 
no tolerances are specified. Therefore, when a dimension is 
given without a tolerance, the degree of accuracy secured in a 
finished part may depend upon the judgment of the man 
doing the work. In order to avoid any misunderstanding 
regarding the degree of accuracy necessary for various classes 
of machine parts, tools, etc., the allowable errors or tolerances 
are usually given on the drawings of work which is considered 
in the precision class. The general practice, however, is not 
to specify tolerances for the rougher or less accurate work, 
although a few concerns give tolerances even though the work 
may be quite inaccurate. 

What Tolerances are Based on. — In dimensioning drawings 
which require tolerances, it is necessary to decide in the first 
place what tolerances should preferably be allowed. This is 
a branch of work which requires considerable experience, and 
a draftsman or designer who has spent many years making 
drawings for interchangeable work might not be able to specify 
the proper tolerances, especially for some new form of mechan- 
ism, because the tolerances depend upon the class of mechan- 
ism and the degree of accuracy necessary in order to secure 
satisfactory operation. Very often draftsmen, especially if 
inexperienced, specify tolerances that are much smaller than 
they should be, because this is considered safe practice. The 
difficulty with this method is that the cost of manufacture is 
often greatly increased without improving the quality of the 
product, because when the tolerances are not so large that 
they interfere with the operation or durability of the mechan- 
ism, they do not affect the quality. 

In view of the fact that tolerances depend upon the nature 
of each part and its purpose, the man in an organization most 



I/J.O MECHANICAL DRAWING 

competent to decide what tolerances should be allowed for a 
new mechanical device, might be the chief draftsman, the 
shop superintendent, or someone else. In any case the deci- 
sion should not be regarded as final until the actual operation 
of the mechanism has demonstrated that the parts are neither 
too loose nor unnecessarily accurate. The exact methods of 
machining and gaging each piece are other factors which 
enter into this problem and make it more complex. The 
draftsman should understand why tolerances are allowed, 
what they are based on, and how they should be indicated on 
drawings, even though he may not always be competent to 
decide what tolerances should be adopted. 

It is often assumed that draftsmen who have had consider- 
able experience and who know something about shop practice 
are capable of specifying the tolerances and that the tool and 
gage designers can use these same tolerances without modifica- 
tion. It is not practicable, however, for a draftsman, or even 
an experienced designer, to give the proper tolerances in all 
cases, especially if some new form of mechanism is being 
designed. If the draftsman specifies tolerances that are too 
small, the mechanism may operate satisfactorily, but in order 
to machine the work to the degree of accuracy specified, im- 
practicable machining and gaging methods might be neces- 
sary, thus greatly increasing the cost of production. Many 
manufacturers who did government work during the war had 
great difficulty because tolerances were unnecessarily small. 

Relation between Tolerances and Manufacturing Methods. 
— As the determination of suitable tolerances is of great 
importance in manufacturing all machinery and tools, accord- 
ing to the interchangeable plan, this subject should be 
thoroughly understood. The first principle to bear in mind 
is that the tolerances are directly related to the purpose and 
action of the mechanism and that they should be as large as 
possible and not unnecessarily small. When tolerances are 
established with this principle in mind, the manufacturing 
costs, to say nothing of the initial cost of special tools, can be 
greatly reduced. 



DIMENSIONING WORKING DRAWINGS 141 

While the tolerance must be based primarily upon the pur- 
pose and action of the mechanism, it is also essential to con- 
sider the methods of machining and gaging. While tolerances 
may be given for some dimensions, such as the sizes of holes, 
shafts, screw-thread diameters, or other work which will be 
located for machining and gaged by methods known before- 
hand, it is impracticable to say what the tolerances should be 
for more complicated work until the exact method of gaging 
and machining it has been decided. The procedure outlined 
in the following has been employed to advantage by the Pratt 
& Whitney Co., in making drawings for accurate work: After 
a mechanism has been designed to perform whatever function 
is required, the design is not considered complete, but is ana- 
lyzed to see if it cannot be changed so as to reduce the cost 
of manufacture without impairing the effectiveness of the 
working of the mechanism. When the design has been simpli- 
fied as much as it can be without interfering with the func- 
tioning of the device, other modifications may be made if 
they will permit the use of better tools or gaging methods. 

The assembly and detail drawings for very accurate work 
have the general dimensions but without tolerances, except, 
possibly, temporary ones to give the tool and gage designers a 
general idea of what is required. In some instances, a cer- 
tain tolerance might be absolutely necessary in order to insure 
the proper operation of the device, in which case it would be 
placed on the preliminary drawing. The tool and gage de- 
signers cooperate with each other until all the tool and gaging 
equipment has been designed or selected. It is essential in 
connection with this work that the dimensions be related to 
points on the work that are naturally adapted for gaging and 
locating purposes. When all of the tolerances have been de- 
termined by considering the functioning of the device and 
practical means of machining and gaging it, these tolerances 
are placed upon the tool drawings. 

Working Drawings without Tolerances. — A great many 
working drawings have the dimensions given in inches and 
fractional parts of an inch, but without tolerances. On such 



142 MECHANICAL DRAWING 

drawings common fractions are used, such as J, J, f, ^, ^, 
or glj inch. Examples of this method of dimensioning are 
shown in Figs. 2 and 3. One not familiar with drafting prac- 
tice might infer that a part dimensioned in this way was to 
be made as closely as possible to the given dimensions, and 
that when less accuracy were required, this would be indicated 
by specifying tolerances. But, according to the general cus- 
tom, when the dimensions are expressed by common fractions, 
it is understood that no great degree of accuracy is required, 
and when drawings are for parts which must be accurate and, 
perhaps, interchangeable, decimal fractions are then used and 
the tolerances are generally given. 

The objection to the first method is that it is indefinite and 
allows considerable latitude for individual judgment, unless 
it is understood that a certain error is allowable when no 
tolerance is given. Even if a length need not conform to a 
given dimension within f inch, there is good reason for specify- 
ing this tolerance on the drawing, because the workmen will 
then know what is required, but at the present time this is 
not the general practice when the drawings are for work 
which is not considered in the precision class. 

The general rule of the Taft-Peirce Mfg. Co. is that, if a 
fraction is not accompanied by a tolerance, a minimum toler- 
ance of plus or minus 0.010 inch is permissible. If the dimen- 
sions must be within closer limits, then the tolerance should 
be specified. In order to make it unnecessary to give toler- 
ances in all cases, it is further understood that two-place deci- 
mals have no tolerances added, if a tolerance of plus or minus 
0.005 i ncn is allowable. Three- or four-place decimals are 
used only when absolutely necessary, and if a tolerance is not 
added it is understood that =*= 0.0015 inch is permissible. The 
allowance of o.oioinch tolerance for all fractional finished 
dimensions has been adopted by a number of large manufac- 
turers. Another general rule is to allow 0.003 tolerance for 
dimensions given in decimals, unless a tolerance is specified. 

Methods of Designating Tolerances on Drawings. — Toler- 
ances may be designated on drawings in several different 



DIMENSIONING WORKING DRAWINGS 143 

ways. A very common method, but one which is not recom- 
mended, is first to write down the basic dimension or the 
one that is considered theoretically correct, and then place 
after it a decimal representing the allowable error above and 
below the basic dimension. For instance, if the basic dimen- 
sion, as, for example, the diameter of a shaft, is 2 inches, and 
a total error of 0.006 inch is considered allowable, the dimen- 
sion would be written 2 ± 0.003, winch means 2 inches plus 
or minus 0.003 inch. 

Another common method is to give the minimum and 
maximum dimensions direct; thus, the dimension just re- 
ferred to would be written J ( , which shows that the 

( 2.003 ' 

minimum diameter is 1.997 inches and the maximum, 2.003 
inches. A third method is to give the tolerance as a certain 
amount either above or below the basic dimension, instead of 
allowing both a plus or minus tolerance. For instance, the 

dimension 2 ' „ indicates that the size may not exceed 2 
— 0.000 

inches, but it may be 0.006 inch less. While dimensions are 
written in this way, there is no good reason for placing the 
-f- 0.000 after the basic dimension, and a simpler way is simply 
to give the tolerance with either a plus or minus sign, as the 
case may be; thus, 2 + 0.006 inch indicates that the diameter 
must not be less than 2 inches and may vary from 2 to 2.006 
inches, or if the dimension were 2 — 0.006, this would show 
that 2 was the maximum dimension instead of the minimum. 

When the tolerances are given as a certain amount above 
or below the basic or nominal dimension, difficulty is often 
experienced in checking a drawing and also in connection 
with the gaging and manufacture of precision work, and it is 
preferable to have all of the tolerances on one side of the basic 
dimension. Then the total tolerance for any number of ad- 
jacent parts or sections can be determined by simple addition. 
This method has been objected to on the ground that it elimi- 
nates the use of standard reamers, because of the necessity in 
some cases of having a hole larger than a standard sized reamer. 



144 MECHANICAL DRAWING 

The number of so-called standard reamers in use, however, is 
small as compared with the number of special reamers, and if 
a reamer is made special, it can easily be made to conform 
with whatever diameter is required. 

Tolerances Indicated by Number of Decimal Places. — 
Still another method of indicating tolerances is based on the 
number of decimal places in the dimension. The following 
system has been adopted by a large manufacturing concern, 
and the same general method has been employed to a limited 
extent by other manufacturers. This method is explained 
because draftsmen often find it necessary to work from draw- 
ings made outside their own plant, and it is desirable to know 
the different systems that are in use. This particular method, 
however, is not considered entirely satisfactory, because it is 
not self-explanatory. 

Figures carried to one decimal place indicate that the total 
limit is o.i or ± 0.05. Figures carried to two decimal places 
indicate that the total limit is 0.0 1 or ± 0.005. Figures carried 
to three decimal places indicate that the total limit is 0.00 1 or 
=*= 0.0005. Figures carried to four decimal places indicate 
that the total limit is 0.0001 or =±= 0.00005. 

When a dimension is followed by the plus sign (+), it indi- 
cates that any variation from the given dimensions must be 
over size within the total limit. 

When a dimension is followed by the minus sign ( — ) , it in- 
dicates that any variation from the given dimension must be 
under size within the total limit. 

When a dimension is followed by an " approximation sign" 
(ap), it is to be understood that any reasonable limit of varia- 
tion will be permitted. 

When the variation permissible is different from that just 
given, the dimension is to be followed by the plus and minus 
sign (±), the plus sign (+), or the minus sign ( — ), and the 
amount of limit, thus: 

1.2 ± 1 indicates that total limit is 0.2 or, plus or minus 0.1. 

1.82 ± 2 indicates that the total limit is 0.04 or, plus or 
minus 0.02. 



DIMENSIONING WORKING DRAWINGS 1 45 

1.258 ± 5 indicates that the total limit is 0.0 10 or, plus or 
minus 0.005. 

1.0937 =*= 3 indicates tha f the total limit is 0.0006 or, plus 
or minus 0.0003. 

1. 18 7 5 ± 20 indicates that the total limit is 0.004 or, plus 
or minus 0.002. 

1.3906 + 30 indicates that the dimension must not be 
under size, but may be 0.003 over s i ze - 

0.1285 — 20 indicates that the dimension must not be over 

size, but may be 0.002 under size. 

+ 2 
0.281 indicates that the total limit is 0.003 an d that 

the dimension may be 0.002 over, or 0.001 under, size. 

Improved Method of Dimensioning Drawings. — Drawings 
that are improperly dimensioned are not only confusing to 
the draftsman who checks them, but to the workman who is 
responsible for making the parts to the specified dimensions. 
When a part like a shouldered shaft must be made accurately, 
it is common practice to give the distance between each 
shoulder and then specify tolerances above and below these 
basic dimensions; dimensions are frequently included which 
are not actually needed and which do not apply to the parts 
requiring accuracy. The result is that such a drawing is not 
only difficult to check, but the work may actually be spoiled 
without exceeding the tolerances specified, as will be shown 
later by an example. 

The improved method to be described has been used suc- 
cessfully by the Pratt & Whitney Co., in the manufacture of 
many classes of interchangeable parts. This method is based 
upon what is known as the maximum metal dimension. This 
expression simply means the size of dimension representing 
the most metal in whatever part the dimension applies to. 
For instance, the maximum diameter of a shaft is the maxi- 
mum metal dimension, because the larger the shaft the more 
metal. On the contrary, the minimum diameter of a hole is 
the maximum metal dimension, because the smaller the diam- 
eter the more metal. 



146 



MECHANICAL DRAWING 



The difference between the maximum metal dimensions of 
two parts which are assembled, as, for example, a shaft and 
a pulley hub, represents what is known as the initial clear- 
ance. The total clearance space would include the tolerances 
and would equal the difference between the minimum metal 
dimensions. Suppose the maximum metal dimension of a 
shaft was 2 inches, and of the hole in the pulley hub, 2.002 
inches; then the initial clearance is 0.002 inch. If a toler- 
ance of — 0.004 inch is allowed for the shaft and -f- 0.003 
for the hole, then if the shaft happened to be made to the 
smallest permissible size and the hole to the largest size, 
the total clearance would equal 2.005 ~~ 1.996 = 0.009 inch. 



^ 



X 



1.75-0.00; 



1.50-0.003 



< — 3.1 25 - 0.004 ^<2.00 - 0. 0043»|< — 2.75 - o. 004 =>j< (4.150-) - 

<S 12.025-o.oi2 



Machinery 



Fig. 4. Drawing on which the Tolerances or Allowable Errors are given 

When the maximum metal dimensions are given on the draw- 
ing, the clearance can easily be determined, thus assuring 
that there is no interference between parts. A point that 
should be remembered is that the maximum metal dimen- 
sions are the basic or nominal sizes and the tolerance should 
always be subtracted from male parts such as shafts, etc., 
whereas, in the case of holes or female parts, the tolerance 
should be added. 

The use of maximum metal dimensions facilitates the check- 
ing of drawings. The sketch shown in Fig. 4 is dimensioned 
according to this method and, as will be seen, the maximum 
metal dimension for the total or over-all length may be ob- 
tained by simply adding the other maximum metal dimen- 
sions together. The total tolerance may also be obtained 



DIMENSIONING WORKING DRAWINGS 



147 



by simply adding the other tolerances. If one or more toler- 
ances require changing, this can be done very readily without 
changing the basic dimensions, assuming that the initial 
clearances do not require changing. These maximum metal 
dimensions are placed on the preliminary drawings, but the 
tolerances are not given at first, because they must be deter- 
mined later after carefully considering all the factors which 
may affect them, such as the machining and gaging methods. 
The maximum metal dimensions, however, establish the 
initial clearances which are determined at the outset and 
which correspond to the minimum clearances. 



5.125-0-008- 

A 



<&-2.00-o.oo4-5>< — 3.125-0.004 

< 5.125-0.008 » 

B 

Machinery 



Fig. 5. (A) Piece that is properly dimensioned. (B) Drawing which gives 
Unnecessary Dimensions 



Unnecessary Dimensions on Drawings. — When tolerances 
are given on drawings, it is especially important to include 
only those dimensions which are actually required. Further- 
more, it is preferable not to repeat dimensions in the different 
views as is sometimes done. This is particularly objection- 
able when it is necessary to make changes in dimensions, 
because where dimensions are repeated, they are sometimes 
overlooked in making the changes. In regard to the partic- 
ular dimensions to be included on a drawing, a good general 
rule is to give only dimensions with tolerances that repre- 
sent sizes which are actually gaged in manufacturing the 
piece. 



148 MECHANICAL DRAWING 

In Fig. 4, three of the dimensions have tolerances, and a 
fourth one (4.150 inches) is given without a tolerance. While 
this latter dimension does not require gaging, such sizes are 
sometimes convenient to have, but if they are placed on the 
drawing, it is good practice to surround the dimension by a 
line as shown, to indicate that it is merely for reference pur- 
poses. Another method of identifying such figures is to place 
the abbreviation "ref." after the dimension, which shows 
that it is merely for reference. This method has been adopted 
by the United States Ordnance Department. 

How Unnecessary Dimensions Cause Errors. — A practical 
example, illustrating how unnecessary dimensions may result 
in errors, is shown in Fig. 5. Sketch A illustrates a piece 
that is properly dimensioned, assuming that the important 
sizes are the over-all length and the length of the enlarged 
part. As the illustration shows, the tolerance for the shoulder 
is 0.004 i ncn an d the over-all tolerance is 0.008 inch. If a 
drawing of this same piece were dimensioned as illustrated at 
B, this might result in an error. In this case, three of the 
dimensions are given including the length x of the shoulder, 
the length y of the small part, and the over-all length. Now 
suppose in making this piece that the over-all length and the 
length y are gaged and found correct or within the tolerance 
specified. It does not follow that the length x of the shoulder 
will be correct. For example, the over-all length may be 
5.1 1 7 inches, which is the minimum length, and the length y 
may be 3.125, which is the maximum; then the length x will 
equal 5. 117— 3.125 = 1.992 inches, but the minimum length 
specified for this part equals 2.00—0.004 = 1.996 inches; 
hence, this length of 1.992 inches is 0.004 mcn under size. 

To consider another condition, assume that the over-all 
length is 5.125 inches, and the length y is 3.121 inches; then 
the length x will equal 5.125— 3. 121 = 2.004 inches, or 0.004 
inch longer than the maximum allowable length. These errors 
could not occur if only the important dimensions were given 
as shown at A, and the piece were gaged as indicated by these 
dimensions. In actual practice, it might be found more con- 



DIMENSIONING WORKING DRAWINGS 149 

venient to gage dimension y than dimension x, in which case 
the dimension x should be omitted and the tolerance properly 
distributed between the over-all length and the length y of 
the smaller section. If the over-all length is 5.125 and the 
length y, 3.125, length x of the shoulder equals 5.125— 3.125 
= 2.00 inches, which agrees with the maximum length speci- 
fied. Now if the over-all length is made 5.125— 0.002 inch, 
and the maximum metal dimension y is increased to 3.127 
— 0.002 inch, then if the over-all length is minimum, or 5.123 
inches, and y is maximum, or 3.127 inches, length x will equal 
5.123 — 3.127 = 1.996, which agrees with the minimum 
length specified. One difficulty with this method is that it 
makes it necessary to reduce the over-all tolerance consider- 
ably, and the method of dimensioning and gaging indicated by 
sketch A should therefore be followed if practicable. 

Arrangement of Dimensions for Locating Holes. — When it 
is necessary to locate several holes on a drawing, or possibly 
a large number, the dimension lines may be arranged in dif- 
ferent ways; there is no general rule which may be fol- 
lowed under all conditions simply because the dimensions are 
primarily for the use of the machinist or toolmaker, and there 
is a close relationship between the method of dimensioning 
and the method of doing the work. The form or shape of 
the part in which holes are required and the degree of accuracy 
necessary are also points to be considered. A drawing with 
the dimensions arranged in a certain way might be entirely 
satisfactory in one shop or tool-room and not in another, 
where the measuring or tool equipment is different. A few 
simple examples will illustrate the principle underlying dif- 
ferent methods of dimensioning and the relation between these 
methods and the work of the shop man. 

The diagram A, Fig. 6, shows a rectangular plate which is 
dimensioned according to the following plan: First the prin- 
cipal hole a is located from the adjacent edges of the plate. 
From this main hole a (which might be any hole that is con- 
sidered the most important) the other holes are located by 
dimensions placed between horizontal and vertical center 



ISO 



MECHANICAL DRAWING 



lines intersecting the different holes. (The diameters of the 
holes have been omitted in this and the other illustrations so 
that the different methods of arranging the hole-locating 
dimensions will stand out more clearly.) 

While dimensions arranged as shown at A enable each hole 
to be located, it is necessary to add the dimensions to obtain 
some center-to-center distances as, for example, the distance 



o 

IO 


-^-3.50-^ 


«s2.25» 


-e2.25^- 




i 

\ 


i 


f 


^ 


._£ 


^ 


I 


1 o 
iq 


% 


a 


\ 


i m 

CM 

1 T^ 




f 




ft 


1 
\ 


h 


f 


1 

fc 


K- ^ 


R 


< 


D 


k 










He— 3.60— 3. 



«s 4.5 



5.75- 

-3.50— 2J 
^2.50^1 



^2.25 



1 

10 



1 



m 



A A A A 



C 



Machinery 



Fig. 6. Different Methods of Dimensioning for Locating Holes 



between the center lines of holes a and b. This might be 
considered an objection on a drawing used in the manufacture 
of duplicate parts, but of little consequence when a drawing 
is used by a toolmaker in the production of one or, at most, 
a few plates. Opinions differ in regard to points of this kind, 
which accounts in part for the various methods of dimension- 
ing found on drawings made in different drafting-rooms. 



DIMENSIONING WORKING DRAWINGS 151 

The advantage of the method shown at A is that the number 
of dimensions used is reduced to a minimum and if changes 
are to be made, the work is simplified; both of these features 
are very important, especially when a drawing is complex and 
represents a part having a large number of holes located irregu- 
larly. The drawing at A is also clear and not likely to be 
misread. 

A modification of the method shown at A is illustrated at 
B. In this case, additional dimensions have been placed on 
the drawing, the object being to give all center distances 
directly so that addition of dimensions will not be necessary. 
Many draftsmen would contend that this method of dimen- 
sioning is superior to that shown at A, but in making such 
comparison the purpose of the drawing should be considered. 
While dimensions for locating only four holes might prefer- 
ably be arranged as shown at B, if there were many holes the 
drawing would be needlessly complex and confusing, because 
of the use of both over-all and center line- to- center line 
dimensions. 

The method of dimensioning shown at C differs to some 
extent from the others referred to, as all dimensions are direct 
from vertical and horizontal center lines intersecting the 
principal hole. For instance, the horizontal distances from 
the vertical center line to holes b, c, and d are given and also 
the vertical distances from the horizontal center line to holes 
b, c, and d. A method of dimensioning master plates, which 
is similar in principle, will be dealt with later. 

Base-line Method of Dimensioning. — A fourth method of 
dimensioning, shown at D, Fig. 6, is sometimes called the 
"base-line" method, because all dimensions are direct from 
two finished edges, each of which serves as a base. This 
method is applicable when the work has finished edges at 
right angles to each other, and plates of irregular shape may 
be mounted on an auxiliary plate having such edges. The 
dimension lines of all four methods that have been referred 
to are at right angles to each other, and in no case have dimen- 
sions been given to show the center distance between the 



152 MECHANICAL DRAWING 

holes in a straight line. This right-angle method, as it might 
be called, is practicable for many classes of work, because the 
adjustments made by the machinist or toolmaker when locat- 
ing the work for drilling and boring are frequently in two 
directions at right angles to each other, the machines or other 
work-holding equipment being arranged in this way. The 
direct center-to-center distance, however, is often required, 
which indicates that the method of arranging the dimensions 
on a drawing may be very closely related to shop and tool- 
room practice. 

Basing System of Dimensioning on Shop or Tool-room 
Methods. — The relation between the method of dimension- 
ing and the particular method of doing the work in a shop 
or tool-room is an important point for the draftsman to con- 
sider. If a number of holes are to be drilled and bored in a 
plate, the latter may be located for each hole by adjusting 
the machine table in two directions at right angles to each 
other. These adjustments can be made quite accurately on 
a good machine having graduated dials on the feed-screws, 
although the latter are subject to wear and new feed-screws 
differ considerably in regard to accuracy. Special vernier 
scales are also used at times to secure more accurate adjust- 
ments than would be possible with a plain scale. For in- 
stance, these verniers are sometimes applied to milling 
machines which are used for boring jig plates or other pre- 
cision work. The common methods of locating work of the 
precision class, however, is by either the so-called button 
method, the size-block method, or the disk method. When 
these methods are employed, the accuracy obtained does not 
depend upon the adjustment of a machine slide which may 
be controlled by an inaccurate screw. 

Briefly, the button method consists in attaching to the work 
very accurate cylindrically shaped buttons or bushings which 
are held in place by small screws tapped into holes that are 
smaller than the holes required, and are located approxi- 
mately where the holes are needed. These buttons are first 
set very accurately according to the dimensions given on the 



DIMENSIONING WORKING DRAWINGS 1 53 

drawing; then one button after another is set exactly con- 
centric with the spindle of a bench lathe or any suitable ma- 
chine, by using some form of indicator, and the different holes 
are bored in successive order. The use of buttons enables 
the toolmaker to locate the holes very accurately because he 
can measure by a micrometer or vernier caliper just how far 
the buttons are apart, so that the method is positive. The 
relation between this button method and the system of dimen- 
sioning will be considered a little later. 

The size-block method consists in adjusting the plate in 
two directions at right angles to each other, and standard size 
blocks are used to determine the amount of adjustment instead 
of relying upon a machine table, even though the latter may 
have a graduated dial reading to thousandths of an inch. 
One method of using size blocks is as follows: Two parallels 
are accurately located at right angles to each other and the 
size blocks are placed between the parallels and the edges of 
the work to secure whatever horizontal and vertical adjust- 
ments are necessary. If the plate does not have square 
finished edges, it may be mounted on an auxiliary plate and 
the size blocks are then inserted between the parallels and 
this auxiliary plate. If the work is being bored in a lathe, it 
is held against an accurate faceplate, and after each hole is 
drilled and bored, the plate is shifted in two directions at 
right angles to each other by inserting size blocks conforming 
to the required dimensions. The draftsman should under- 
stand, at least in a general way, these locating methods, par- 
ticularly if he is to make drawings of precision work that 
must be located by them. 

The disk method, which is often used in connection with 
watch work and other small precision work, is so called be- 
cause the holes are located by means of disks instead of but- 
tons. These disks are accurately ground to such diameters 
that when their peripheries or edges are in contact, the center 
of each disk will coincide exactly with the position of the hole 
to be bored. In order to locate a hole, the disk — which is 
temporarily attached to the work — is first set exactly con- 



154 MECHANICAL DRAWING 

centric with the lathe spindle by using a test indicator, and 
then this disk is removed for drilling and boring the hole. 
The other holes are then located in the same manner. These 
different methods of locating work have been referred to be- 
cause they are closely related to different methods of dimen- 
sioning drawings, as will be more apparent after studying the 
following examples. 

Considering now the relation between the method of dimen- 
sioning and the method of doing the work, it will be apparent 
that if a jig plate were dimensioned as shown in Fig. 6 it 
would be adjusted in two directions at right angles to each 
other for locating the holes, because horizontal and vertical 
dimensions are given. If the work were located simply by 
adjusting the table of, say, a milling machine and the hole a 
were drilled and bored first, the table might then be adjusted 
horizontally and vertically to correspond . to the location of 
hole b. Then it would again be moved horizontally and ver- 
tically for holes c and d, respectively. 

If the drawing were dimensioned as at C, the vertical and 
horizontal center lines could each be considered as the zero 
line and all adjustments be made directly from them. Sup- 
pose, for instance, that the adjustments of the machine table 
were used, the movements being indicated by graduated dials. 
If these dials were set at zero when the machine was properly 
adjusted for boring hole a } the adjustments for all of the other 
holes could then be obtained by direct readings, and this 
same principle would apply in case the size blocks were used. 
A drawing dimensioned as at C would be more convenient 
than one dimensioned as at A, because the former gives the 
dimensions relative to the zero positions. 

Center-to-center Dimensions for Locating Holes. — While 
any of the drawings illustrated in Fig. 6 could be used for 
locating the work, either by adjusting the compound slides 
of a machine table or by means of the size-block method pre- 
viously described, if the button method were employed, the 
dimensions could be arranged so as to be more convenient 
and useful to the toolmaker, provided it were considered neces- 



DIMENSIONING WORKING DRAWINGS 



155 



sary to check the center distances by direct measurements, 
which is usually the case with precision work. If the work 
had two finished edges at right angles to each other and the 
base-line method of dimensioning (illustrated at D, Fig. 6) 
were practicable, the buttons could then be set easily by plac- 
ing the work on a surface plate with first one edge and then 
the other in contact with the surface plate, so as to measure 
in both directions by using a vernier height gage. As the total 
distance to each hole is given, the height gage could be set 




Fig. 7. Drawing on which Center-to-center Distances between Holes are given to 
avoid Calculations in Tool-room 

by referring directly to the dimensions. But after setting 
the buttons by this method, the toolmaker would have to cal- 
culate the center-to-center distance between the holes, if it 
were considered necessary or desirable to check the location 
of the buttons by direct measurement. Any of the drawings 
shown in Fig. 6 give the lengths of two sides of a right-angle 
triangle, from which the length of the hypotenuse may be 
determined in order to check the center-to-center distance by 
direct measurement. 

The drawing shown in Fig. 7 has these center distances, 
and for this reason is more convenient for the toolmaker when 



156 MECHANICAL DRAWING 

measurements over the buttons are required. These center 
distances might, of course, be included on drawings dimen- 
sioned according to any of the methods illustrated in Fig. 6. 
When checking the distance between two buttons, the usual 
method is to measure the over-all dimension or the distance 
from the outside of one button to the outside of the other. 
The center-to-center distance, however, should always be 
given on the drawing. The toolmaker then adds to this di- 
mension an amount equal to the diameter of whatever size 
buttons are used, all buttons for any one job being of the 
same diameter. The center-to-center distances are also needed 
when the holes are located by the disk method. If a jig plate 
or master plate has a large number of holes, the center dis- 
tances of only those holes which must be very accurately 
located might properly be given on a separate view or dia- 
gram, which would show clearly just where great accuracy is 
necessary. A knowledge of the conditions governing each 
case must be considered and the aim of the draftsman should 
be to help the shop man whenever possible even though this 
involves departing occasionally from customary practice. 

Cross-slide and Angular Methods of Dimensioning. — Two 
methods of dimensioning drawings, known, respectively, as 
the "cross-slide" and " angular" methods, are sometimes 
utilized in connection with drawings of master plates or other 
precision work, particularly when it is necessary to locate a 
series of irregularly spaced holes. Master plates are often 
used to insure locating, with great accuracy, a number of 
holes in several duplicate plates. These master plates are 
used by watch manufacturers and also by toolmakers and 
model-makers. Two methods of dimensioning which have 
been applied to the making of watch master plates, are illus- 
trated diagrammatically in Fig. 8 which illustrates the prin- 
ciples involved. 

The cross-slide method of dimensioning is shown at A. As 
will be seen, the drawing has horizontal and vertical center 
lines and each hole is located relative to these center lines by 
giving the horizontal and vertical dimensions. These dimen- 



DIMENSIONING WORKING DRAWINGS 



157 



sions, in each case, represent the total distances from the 
center lines to the hole. When the dimensions for the master 
plate are given in this way, it is because the draftsman knows 
that the plate is to be located for boring each hole by adjust- 
ing it in two directions at right angles to each other. In one 
of the large watch factories, a special bench lathe faceplate 
having compound slides is sometimes used for drilling and 
boring master plates. The cross-slides are located at right 




Fig. 8. Two Methods of dimensioning Master Plates such as are used in 
Watch Manufacture 



angles to each other and are equipped with micrometer screws 
which afford a rapid and accurate method of adjusting the 
plate. 

The angular method of dimensioning shown at B in Fig. 8 
is considered superior to the cross-slide method in some cases. 
All holes that are the same distances from the center are con- 
nected by an arc, and the location of each hole is determined 
by the radius of its arc and the angle in degrees and minutes 
between the hole and a vertical center or zero line. The 
fixture for holding the master plate dimensioned in this way 
must, of course, be arranged both for radial and angular ad- 
justments, and if considerable accuracy is necessary, a vernier 
should be used in conjunction with the degree graduations. 



158 MECHANICAL DRAWING 

Some of the holes in watch plates do not need to be located 
with the degree of precision required in some other classes of 
work, and when there are several series of holes each located 
on the same arcs or at the same radial distances, the angular 
method of locating them is very convenient. For example, 
if two holes are located 0.293 mcn from the center and an- 
other series of four holes is located 0.495 mcn from the center, 
the plate is first adjusted radially 0.293 and is then set to cor- 
respond to the angles between the two holes on this arc. If 
the four holes mentioned are the next in order, the slide is 
adjusted radially to the 0.495 position, and then in an angular 
direction for locating each of the holes on this arc. The same 
procedure is, of course, followed for all of the other holes, 
and this is a very convenient and rapid method for the class 
of work to which it is applicable. 

There are certain holes in watch master plates which are 
not located by either the angular method or the cross-slide 
method, as a more positive and accurate way is required. 
Such dimensions as the "dep things" of the train and the es- 
capement usually require locating a certain hole accurately 
from two other holes or points. These dimensions may be 
secured from some other master plate or model, or they may 
be arrived at by computation. For operations of this kind, 
the location of the master plate for drilling and boring is fre- 
quently controlled by means of the disk method, which has 
already been referred to. When the button method is em- 
ployed, as in the case of larger master plates than are used for 
watch work, the center-to-center distances should be given 
on the drawing, as mentioned before, and a separate view 
may be preferable containing only these center dimensions, 
particularly if there are many closely spaced holes and other 
dimensions with little room for all of them on one view. 

Designating Angles and Tapers. — The dimensions of 
angles may be given in degrees and minutes, in which case 
the dimension line is an arc with its center usually at the inter- 
section of the two lines forming the angle, as shown at A in 
Fig. 9. If the angle of a conical part is given, it may be the 



DIMENSIONING WORKING DRAWINGS 



159 



included or total angle as at B or one half the included angle, 
which is the inclination of one side relative to the axis as at 
C. The angle, however, is not always given relative to the 
axis as will be explained later in connection with Fig. 10. 

The inclination of a tapering plug or hole might be expressed 
either in degrees, in inches of taper per foot, by a name and 
number, or by giving the diameter of the large and small 
ends of the tapering part. The practice varies considerably 
and depends somewhat upon the amount of taper and whether 
or not it conforms to some standard. When the taper is very 
small, the taper in inches per foot is often given and the diam- 
eter at one end. The tapers of tool shanks and sockets are 



-A 


A 
XJo^\ 


4\ 

/A 




/ 




A 


1 

B 


1 
c 

Machinery 



Fig. 9. Designating Angles on Drawings 

indicated by name and number. For instance, if the tapering 
end or shank of a reamer were marked a No. 4 Morse taper," 
this would mean that the No. 4 size of the Morse standard 
was required. The Brown & Sharpe and Jarno are other 
well-known standards for tapers. The various numbers and 
their corresponding dimensions are listed in engineering hand- 
books. Gages which conform to the different numbers are 
ordinarily used in shops handling work of this kind, so that 
it is not necessary to give the actual dimensions on the 
drawing. 

Sizes of taper pins, such as are used for doweling parts 
together or for securing collars or hubs to their shafts, are 
also designated by numbers. These numbers for the generally 
accepted standard range from No. o to No. 10, and sizes 



i6o 



MECHANICAL DRAWING 



smaller than No. o are, according to the practice of the largest 
maker of taper pins in the United States, indicated by zero 
marks varying from No. oo to No. oooooo. These sizes below 
No. o, however, do not conform to a universally recognized 
standard. Incidentally the numbers indicate the diameters 
of pins at the large ends and the length of the pin varies to 
suit conditions. 

Specifying Angles that Conform to Machine Graduations. — 
The preferable method of designating angles depends some- 





\ 

\ 
) 

1 








\ 
i 
i 


\ 


^ 




1 







\ 33 


I 




9 


i_ 






/ 

/ 




62 


/ 
/ 


/ 


r 




/ 


V 


B / 

T28 


<J>7 


Machinery 



Fig. 10. (A) One Method of giving the Face and Edge Angles of a Bevel-gear Blank. 

(B) A Drawing of the Same Bevel-gear Blank with Angles given which 

conform to Machine Graduations 

what upon the way the work is to be handled. As a general 
rule, angles should be given which correspond to the angular 
graduations on whatever machine parts need to be adjusted 
for machining the tapering surfaces. This point is illustrated 
by a comparison of sketches A and B, Fig. io, which represent 
a bevel pinion blank. On sketch A the angle of the conical 
face, relative to the axis, is given as 33 degrees, which is one 
half the included angle, and the inclination of the beveled 
edge is given with reference to a line parallel with the axis. 
If this blank is to be turned on an engine lathe, the compound 



DIMENSIONING WORKING DRAWINGS 161 

slide would not, of course, be set to 33 degrees, but to 57 
degrees when turning the conical face, and if the bevel edge 
is also turned by means of the compound slide, the latter 
would be set to 28 degrees, and not to 62 degrees. If these 
angles were given as illustrated at B, they would then corre- 
spond to the angular position of the tool-slide as represented 
by its graduations, which is preferable for the shop man. 
While it is a simple matter to determine what the position of 
the compound slide should be when the angles are given as 
shown at A, the fact remains that many machinists are 
puzzled and are obliged to ask questions when the angle on 
the drawing does not show directly the angular position of a 
machine slide. 

Angles are given in degrees and minutes when a fractional 
part of a degree must be indicated on the drawing. Thus 
14! degrees would be written 14 20'. If the graduations on 
a machine tool are in degrees, it is possible, of course, to judge 
one half or one fourth of a degree quite closely. For in- 
stance, if the angle specified were 74 degrees 45 minutes and 
the machine graduations were in degrees only, which is the 
common method of graduating on ordinary machine tools, 
the tool-slide could be set to 74 degrees plus three fourths of 
a degree quite closely, but when angular work is in the pre- 
cision class, as in toolmaking and gage-making practice, either 
a gage or a sine bar is employed instead of relying upon the 
setting of a machine slide, although some machines and fix- 
tures designed for accurate work have vernier attachments 
for the angular graduations. 

Tabulated Drawings. — In plants where small tools or rela- 
tively simple mechanical devices are manufactured, which 
vary in regard to size but are all of the same general form, 
what are known as " tabulated drawings" are used. A draw- 
ing of this kind does not have the dimensions marked directly 
on it, but letters are used instead and the corresponding di- 
mensions are given in a table which accompanies the drawing. 
Such drawings are not as convenient to work from as those 
having the dimensions on them and errors are more easily 



l62 



MECHANICAL DRAWING 



made with tabulated drawings, so that this plan has not been 
adopted very generally for shop or working drawings. The 
tabulated drawings, however, are convenient for reference 
purposes. An example of a tabulated drawing is shown in 
Fig. ii. This drawing represents three views of a pivoted 
clamp intended for the use of toolmakers. This clamp is 
made in four different sizes, as indicated by the numbers to 



|< A- 

D 







Size 
No. 


A 


B 


c 


D 


E 


F 


G 


H 


I 


J 


K 


L 


i 


3 


2% 


% 


2}{ 


% 


% 


%6 


I 


iK 


3 / 8 


% 


X 


2 


3 


2% 


\ 


2% 


i/ 
h 


% 


%S 


I 


iM 


% 


% 


X 


3 


i 


lYi 


% 


1% 


h 


% 


%6 


I 


iH 


% 


% 


% 


4 


2K 


iK 


% 


2}i 


}i 


% 


%B 


I 


iK 


% 


% 


% 



Fig. 11. Tabulated Drawing for Toolmaker's Clamps of Different Sizes 



the extreme left in the table, and the figures beneath the dif- 
ferent letters represent the dimensions corresponding to these 
letters on the drawing. 

Tabulated Tool Drawings. — In all large factories doing 
considerable tool work, as in the automobile line, there are 
many tool drawings which are of similar design but of varying 
dimensions. These tool drawings are principally of small 
tools, such as reamers, taps, arbors, gages, etc. Various 



DIMENSIONING WORKING DRAWINGS 163 

methods are in use to prevent a duplication of drawings and 
at the same time provide the shop with adequate information 
for manufacturing the tools. One of the most simple of 
these in common use is to have a large sheet, showing at the 
top a drawing of the tool wanted, with letters designating the 
various dimensions. The tabulations are filled in with all 
the necessary dimensions and give complete information as to 
sizes; then, when an order is issued for any tool shown on 
the drawing, the entire sheet is blueprinted and all entries 
are crossed out except those required. 

This method, however, has its disadvantages, one of which 
is that in a short time the sheet becomes so crowded with 
figures that a draftsman, in looking it over, may pass by the 
dimension wanted and make an additional entry through 
failure to note the original. Another difficulty is that a drafts- 
man cannot picture in his mind's eye just how the tool will 
look when completed. This is a serious fault, which has 
resulted in several freakish and unworkable tools being made. 
The tabulations also confuse the toolmakers and are objection- 
able to most of them. 

Another system in vogue is to use the master tabulated 
sheet in the tool designing department only, and a blocked-in 
blueprint in the tool-room. Then, when an order is issued 
for the tool-room, the various dimensions are copied from 
the master tabulated sheet and entered on the corresponding 
blank spaces on the blueprint which is then sent to the tool- 
room as a working drawing. The confusion caused by the use 
of the previously mentioned system is eliminated when this 
method is used, but the other disadvantages remain, with the 
additional fault that mistakes are likely to be made in copy- 
ing the entries on the print. These could be prevented by 
checking, but this entails additional work. Again, on some 
of the tools it is necessary to note an exceptional treatment, 
such as hardening, grinding, polishing, etc. To mark this on 
the master sheet would require a footnote or something similar. 
In filling out the blocked-in prints, these notes are sometimes 
ignored, resulting in an incorrect tool. Another drawback to 



164 



MECHANICAL DRAWING 



the system is that it is necessary to search for a desired entry, 
owing to the fact that the entries are recorded in the order 
of issuing them and not according to size, which is the infor- 
mation needed by the drafting-room in looking up data for 
any new work. 

Individual Tracings of Tools. — In order to eliminate all 
the objections and at the same time maintain a correct record 
of tools of this kind, the following system has been established 
with good results in one of the country's largest automobile 
factories. An individual tracing (see Fig. 12) is made of 



ILf" 

JL 



I 5.00 0.015 

I 0.031 . 

1 1.50 '0.031 1 

1 FLATS FOR STAMPING 
STANDARD LOCATION 




ii 



GENERAL USE 



L 1. 5302-1. 5322 DOUBLE PLUG 



B7397 



THE PIERCE-ARROW 
MOTOR CAR CO 
BUFFALO, N.Y. 



Machinery 



Fig. 12. Drawing of a Plug Gage 

each tool, and although this may seem unnecessary, it is 
really not as big a proposition as it appears, as most of the 
drawings are simple. In the lower right-hand corner above 
the drawing number of the individual sheet, the number of 
the master sheet is recorded. The important dimensions 
(which are usually the diameter or length) as well as the 
number of the individual drawing and the distinguishing 
mark of the tool, are entered on a slip of paper and placed in 
a Rand visible file which is used as a permanent record. 

The construction of the panel units in this file is such that 
the celluloid tubes can be readily moved about and replaced 



DIMENSIONING WORKING DRAWINGS 165 

or removed entirely, as desired. The units used are hung on 
rings erected on a central standard. These rings have a num- 
ber of holes in which the hinges of the panel units swing. 
The rings can be changed and larger ones substituted, so that 
more panels can be used when necessary, and expansion can 
thus be readily taken care of as the index grows. The method 
of making the entries is extremely simple, as they can be 
made in less than a minute on the typewriter and are easily 
read. The slips are filed in order of size, so that any entries 
can be found in a minimum amount of time. Should an 
entry be canceled, it is a slight matter to remove the slip and 
advance all remaining entries to fill the vacant space. This 
system works out very well without any of the disadvantages 
incident to the method mentioned in the foregoing and, being 
capable of expansion, it may be continued indefinitely. 

It has been found by experience that in order to have tools 
made that are suitable' for manufacturing a certain part, it is 
necessary for the draftsman to make an individual drawing 
of each, in order to obtain the correct proportions. This 
necessitates at least a pencil drawing to scale, from which 
the dimensions may be transferred to the master sheet. To 
make a tracing of this tool drawing does not require more than 
an hour or so. In a factory the size of the one using the sys- 
tem referred to, the number of orders for these so-called tabu- 
lated tools is so large that the amount of time consumed in 
entering the dimensions on a blocked-in print would soon be 
considerably greater than that required to make a tracing. 
These tracings are checked and placed in a file so that they 
can always be used for reference. There is also an added 
advantage if some existing tool is to be used, as this can be 
easily determined by reference to the tracing. This is more 
apparent on manufacturing than on inspecting tools, but as 
the system mentioned covers 106 different types, something 
equally satisfactory for all must be used, which requirements 
are rilled by this system. 

Dimensions on One Drawing for Parts of Different Sizes. 
— When a certain part is to be manufactured in several dif- 



i66 



MECHANICAL DRAWING 



ferent sizes, dimensions for these various sizes are sometimes 
placed on one drawing, instead of using a tabulated drawing 
and to avoid making an original tracing for each different 
size. An example is shown in Fig. 13. This particular piece 
is required in six different sizes, which are designated by num- 
bers 1 to 6. As will be seen, the dimensions for each size are 
numbered to correspond with the number of the part. For 
instance, in this case the over-all length of the smallest size, 
or No. 1, is 3ff inches, whereas the largest size, or No. 6, has 



ft 

2 

7 . 



TT 

to If) 



TTTT 

TTTT 

-f COCJ y 

Y Y Y Y 



°32 

"4ff- 

-4A- 

-2U- 



TTTTTL 

N[cn«|«cn|cor-lco wit,, 



NJco«h°cn|co 

TTTTT 



sis TTTTTT 

I 1 *>|<ax|a>KcD oil* r-|io,.i m 



-|a)N|cor-;(D»>|f. : |<o, D | I 

II I ||| 

CO CM rf 



j^J4*j^J*H*H*t*l 



5— 



-8«- 
"4f 6 - 
-4-I-- 

■BH- 

-6i- 



TTTT 

TTTT 



m-t m to 

■mi. 



Machinery 



Fig. 13. Dimensions on One Drawing for Parts of Different Sizes 



an over-all length of 6f inches. This method is practicable 
for simple parts provided the number of different sizes wanted 
is relatively small. If there were many sizes, difficulty would 
be experienced in finding room for the dimensions, especially 
on certain parts of the drawing where only a small amount of 
space is available. Sometimes, when there is insufficient 
room for the dimensions of interior surfaces, extension lines 
are used to permit placing the dimensions of holes, recesses, 
etc., outside of the limits of the drawing proper, but such an 
arrangement may prove confusing. 

Another way to avoid making more than one original draw- 
ing of different sized parts which are the same, or practically 



DIMENSIONING WORKING DRAWINGS 167 

the same, in design, is as follows: The dimension lines are 
placed on the original tracing, but all dimensions which vary 
for parts of different sizes are omitted. Instead of the dimen- 
sions, solid black circular .dots are drawn on the tracing and 
these form white spaces on the blueprints. A blueprint is 
made for each size, and the correct dimensions for each par- 
ticular size are marked with ink in the blank spaces. The 
object of this plan is, of course, to avoid making more than 
one original tracing. Any dimension or information which 
applies to all of the different sizes is marked on the tracing, 
blank spaces being left only for dimensions which vary. 

Drawings Dimensioned according to Metric System. — 
When drawings are dimensioned according to the metric sys- 
tem of measurement, all dimensions are expressed in milli- 
meters. The reason for not expressing the dimensions in 
decimeters or meters is the possibility of mistakes due to mis- 
placing decimal points. * No matter what the dimension is on 
a drawing showing machinery or machinery parts, the dimen- 
sion is expressed in millimeters. This often means numbers 
of five figures, but the rule is invariable. The practice as re- 
gards scales is to use full -J- and ^ scales; if necessary, -^0 
^0 and x^o scales may be used, but \, \, \ and other binary 
division scales are not recommended. 



CHAPTER VII 

INSTRUCTIONS ON WORKING DRAWINGS AND 
PROCEDURE WHEN CHECKING 

Mechanical drawings frequently require, in addition to 
the complete dimensions, instructions in the form of either 
symbols, abbreviations, or explanatory notes. The object of 
such instructions may be to indicate the kind of finish desired, 
the method of heat-treatment, the kind of thread, the type 
of tool to use, as, for example, whether a drill or a reamer will 
finish a hole accurately enough, and any other information 
needed to make a drawing complete, both as a guide to the 
men in the shop and as a record of how the part is made. 
Abbreviations which have not become standard and are not 
generally understood, are objectionable and it is preferable 
to write out the word or expression so that the drawing will 
be self-explanatory. A draftsman, however, may find it 
necessary to use abbreviations which have been adopted in 
the drafting-room where he is employed, even though they 
may not be understood elsewhere, in case the drawings are 
sent to other manufacturing plants, as is sometimes done. 

List of Abbreviations. — The accompanying list of abbre- 
viations is confined largely to those which have been used 
very extensively. While a few of these abbreviations may 
not be universally understood, nevertheless they are included 
in this list to assist the draftsmen in understanding drawings 
on which they may be found. 

The use of some abbreviations is desirable, especially if 
they must be placed on many different parts of the drawing, 
as is often the case with the so called " finish marks" referred 
to in the following paragraph, and certain other abbreviations 
such, for example, as the U. S. S. for the United States stand- 
ard screw thread. 

168 



INSTRUCTIONS ON DRAWINGS 1 69 

Symbols and Abbreviations Used on Mechanical Drawings 



Bab. 


babbitt 


C. P. 


circular pitch 


Bz. 


bronze 


D. P. 


diametral pitch 


Br. 


brass 


P. D. 


pitch diameter 


Ph. Br. 


phosphor bronze 


Thds. per in. 


threads per inch 


Cop. 


copper 


/ 


finish 


CI. 


cast iron 


R. P. M. 


revolutions per minute 


Mai. I. 


malleable iron 


mm. 


millimeters 


W. I. 


wrought iron 


U. S. S. 


United States standard 


C. R. S. 


cold-rolled steel 


U. S. F. 


United States standard 


T. S. 


tool steel 




form 1 


M.S. 


machine steel 


B. W. G. 


Birmingham wire gage 


s. c. 


steel castings 


A. W. G. 


American wire gage 


S. Forg. 


steel forging 




(Brown & Sharpe) 


S. Tube 


steel tubing 


Hd. 


head 


ft. or ' 


feet 


Fil. 


fillister 


in. or " 


inch 


C. to C. 


center to center 


ins. 


inches 


Sq. 


square 


sq. in. 


square inch 


Hex. 


hexagonal 


sq. ft. 


square feet 


Oct. 


octagonal 


or deg. 


degrees 


L. H. 


left hand 


± 


plus or minus 


R. H. 


right hand 


D, dia. or diam. 


diameter 


Sc. 


screw 


R, or Rad. 


radius 


Std. 


standard 


Thds. 


threads 







1 This abbreviation means that the thread conforms to the United States standard in regard to 
the form of the thread only, the pitch being greater or less than standard. 

Finish Marks. — What are known as "finish marks" are 
placed on drawings to show what surfaces need to be finished. 
The letter "/" is almost invariably used. This letter is placed 
with the cross-bar on the intersection of the line representing 
the surface to be finished. A drawing which has a number of 
these finish marks is shown in Fig. 2, Chapter VI, and illus- 
trations will be found in other chapters. The letter should 
always be placed across the line so that it will be noticeable 
and the cross-line of the letter should preferably intersect 
that line on the drawing which represents the surface to be 
finished. The practice of some concerns is to use the capital 
letter "F" with the foot resting on the line indicating the 
surface to be finished. 

Finish marks are not always placed on drawings, because 
they are considered unnecessary. These marks will frequently 
be found on drawings of cast-iron parts and forgings to show 



170 MECHANICAL DRAWING 

clearly which surfaces are to be machined and which ones 
are to be left rough. Finish marks would be unnecessary on 
many drawings, because it is evident that certain, or all, sur- 
faces require machining. 

Symbols which Indicate Kind of Finish. — It is the prac- 
tice in some drafting-rooms to use symbols which not only show 
what surfaces are to be finished, but indicate the kind of finish 
desired. For instance, the kind of finish may be designated 
by numbers; thus, the expression "No. 1 finish" might mean 
that the surface should be turned; "No. 2 finish," that it 
should be ground; "No. 3 finish," that it should be ground 
and polished; "No. 4 finish," that it should be ground and 
lapped. These numbers are selected arbitrarily and their use 
is not governed by any fixed standard. Numbers have also 
been used in conjunction with the finish marks. For ex- 
ample,/ 1 might mean a ground surface; f 2 , a smooth turned 
surface; / 3 , a polished surface, etc. The use of such symbols 
is not recommended as they are not always understood even 
in the particular shop where the method has been adopted, 
and are especially liable to confuse new workmen. The pref- 
erable way is to write out in plain English exactly the kind 
of finish desired. 

When a part is to have all of the surfaces finished, the ex- 
pression "/ all over" is sometimes placed on the drawing, 
but this has the disadvantage previously mentioned of being 
indefinite. Some contend, however, that the drawing should 
not specify the method or process of obtaining, the finish, 
because manufacturing methods are constantly changing, and 
it is not considered desirable to place instructions on a draw- 
ing which may become obsolete. The consensus of opinion is 
that any disadvantages from this cause are more than offset 
by the advantage of giving definite and specific information 
on the drawing at the time it is made. If machining processes 
are changed later, the drawing can then be changed accord- 
ingly. The kind of finish may be indicated definitely by such 
expressions as "rough-grind"; "finish-grind"; "harden, grind, 
and lap"; "disk-grind"; "mirror finish" (meaning a very 



INSTRUCTIONS ON DRAWINGS 171 

fine polish), etc. These terms are not only definite but will 
be understood wherever the drawings may be sent. 

Explanatory Notes on Working Drawings. — Some of the 
common mistakes made in using drawings could be avoided 
by a more liberal use of explanatory notes wherever mis- 
takes were likely to occur. A drawing should tell its own 
story without the aid of an interpreter. If it needs an inter- 
preter, it is seriously lacking. Standardization of practice in 
the making of mechanical drawings would be desirable, and 
also uniformity in conventions. The use of conventions, 
however, is sometimes misleading, because no matter how well 
they are commonly understood by the drafting fraternity, 
the conditions in modern shops will always lead to mistakes. 
Many men are employed in machine shops who have had no 
mechanical training and, consequently, have never studied 
mechanical drawing. Conventions are Greek to many ma- 
chine operators and they must have the aid of the foreman 
or someone else to explain them. Notes in English are gen- 
erally understandable and should, therefore, be employed in 
preference. 

Explanatory notes are used to designate the kind of ma- 
terial, as, for example, machine steel, tool steel, cold-rolled 
steel, or steel of a certain carbon content. When several 
pieces of one kind are needed, the number of duplicate ones 
required is noted on the drawing. These explanatory notes 
also relate to different kinds of small tools that are to be used, 
such as drills, reamers, and counterbores; thus the number 
or size of the drill and the size of the reamer, if used, are given. 
If punched bolt holes are considered good enough in connec- 
tion with parts made of sheet metal, and close-fitting bolts 
are not necessary, the holes may be marked " punch xe inch 
for |-inch bolts." If holes are to be cored instead of being 
drilled this would also be noted so that the patternmaker 
need not inquire about this point. As screw threads are repre- 
sented on drawings by conventional methods, and do not 
show the kind of thread, this is indicated by a note. For in- 
stance, a hole might be marked "tap f inch U. S. S. thd.," 



172 MECHANICAL DRAWING 

meaning that a U. S. standard tap of f-inch size is to be used. 
The number of threads per inch may or may not be given, 
provided the pitch is standard. When the pitch is not stand- 
ard, the number of threads per inch should always be given. 
If the thread is a U. S. standard form, but is not standard in 
regard to the number of threads per inch, it would be marked 
"1 inch U. S. F., 12 threads per inch." The taper of the 
shank of a tool such as a reamer would, if standard, be desig- 
nated by a number applying to that particular standard, as 
for example, a "No. 4 Morse taper." These examples indi- 
cate the kind of information which is given in the form of 
short explanatory notes. 

Titles on Drawings and Records Required. — Mechanical 
drawings are given titles which are commonly placed in the 
lower right-hand corner of the tracing, usually within a ruled 
inclosure. This title generally includes the name of what- 
ever part or mechanism is shown on the drawing, the firm 
name and address, the drawing number, the date the drawing 
was approved, and the initials or other means of identifying 
those responsible for the drawing, such as the checker and 
engineer who approved the design. A symbol, usually con- 
sisting of a letter and number combined to represent the 
type and size of the machine on which the detail shown on 
the drawing belongs, is often included in the title and other 
identifying numbers and information. 

In Fig. 1 is shown the standard title used on drawings of 
one large plant; these titles are printed on blank sheets of 
tracing cloth, ready for the draftsman. A "T" is placed in 
the lower right-hand corner of the title and in the lower left- 
hand corner of the tracing; this stands for Thurlow (Thurlow 
Works). The drawing number is placed next to this letter, 
so that when finished it will read T-1524, etc. 

The arrangement of titles and the use of symbols, letters, 
etc., varies in almost every drafting-room, and the drafts- 
man should become familiar with whatever system of record- 
ing tracings and machine parts has been established by his 
employer. The system may be very simple or quite complex, 



INSTRUCTIONS ON DRAWINGS 



173 



depending upon the size of the plant and the variety of its 
products. In addition to means of locating and identifying 
drawings, the parts represented by them may require num- 
bering for identification, and lists of parts or bills of materials 
are also in common use. Systems of designating parts by sym- 
bols or numbers and the purpose of part lists are explained in 
Chapter XII. 

Stamping or Printing Titles. — The title of the drawing is 
usually lettered by hand, but to save time and also secure 
neat titles of uniform materials, the title may be stamped or 
printed on the tracing (see Fig. 1). Blank spaces may be left 



REVISED 






AMERICAN STEEL FOUNDRIES 






m ■ 








THURLOW WORKS-CHESTER, PA. 








SPACE FOR TITLE 




























Q 
Ul 

O 




Ul 

a: 



SCALE 


d 

O 
O 


O 

| 
UJ 


O 
UJ 

=> 








DATE 








RECOMMENDED 
















APPROVED 








Works Engineer 








AUTHORIZED 








Works Manager 



Fig. 1. Example illustrating the Title of a Drawing 

for names and -numbers which are filled in for each drawing, 
or the entire title may be set in type and printed when a print- 
ing press is used. The best results are obtained by using a 
printing press for printing the border line and title on the 
various sized sheets of tracing cloth which have been adopted 
as standard. Sometimes an ordinary rubber stamp is used, 
although this method, while simple and inexpensive, is in- 
ferior to printing. One large concern has a stamp with spaces 
for the name of the part drawn, material specifications, heat- 
treatment, if any, required, the initials of the draftsman, 
checker and tracer and a space for the symbol number. 

Another method of avoiding hand lettering on titles is by 
using stencils which are cut out of tin or copper sheets. A 
stiff short brush should be used. In order to do the stenciling 



174 MECHANICAL DRAWING 

neatly, moisten the brush with a little water and rub it along 
a stick of ink until it cannot absorb any more. The brush 
should never be dipped into a saucer of ink or the ink applied 
with a pen. Draftsmen sometimes cut their own stencil 
plates by using a stiff drawing paper and applying a coat of 
varnish to the upper surface. 

Lettering. — Many of the books on mechanical drawing 
have emphasized the importance of lettering, and in most 



A B CDETGH/JKL MNOF> Q 
RSTUVWXYZ 

abode fgh/jk/mnopqrsiuvwxyz 



ABCDEFGHIJKLMNOPQ 
RSTUVWXYZ 

abcdefghijklmnopqrstuvwxyz 
J234567890 1234567890 



Fig. 2. Styles of Lettering used on Most Drawings 

books previously published, considerable space is given to 
this subject and various styles of letters are illustrated, rang- 
ing from the small sizes to large designs which are supposed 
to be drawn accurately by the use of instruments. In actual 
practice, these different styles are seldom, if ever, used. The 
chief requirements are to do the lettering neatly and so that 
all words are legible and easily read. Lettering is usually 
done free-hand by using either an ordinary writing pen or a 
special lettering pen. The slanting Gothic style of letter 
illustrated in the upper part of Fig. 2 is in very general use. 
The capital letters are employed for titles and headings, and 



INSTRUCTIONS ON DRAWINGS 175 

the small letters for notes and instructions, or wherever the 
larger letters are unnecessary. This is known as a " single- 
stroke" style which does not mean that the pen is not lifted 
from the paper in making the letters, but that the line width 
is obtained with one stroke of the pen. With practice, letters 
of this kind can be made quite rapidly without sacrificing 
neatness or legibility. Some draftsmen prefer the vertical 
style of lettering also shown in Fig. 2. The letters are more 
round or open than the slanting Gothic letters but practically 
the same otherwise. The slanting and vertical figures are 
also shown in this illustration. The large a built-up" or 
drawn letters are seen occasionally on the titles of drawings 
— especially large sizes — but in most drafting-rooms the 
drawing of letters mechanically is considered a waste of time. 
When the title is printed or stamped on the tracing, as ex- 
plained in a preceding paragraph, very little lettering is neces- 
sary unless notes are required on the drawing. 

Why Errors on Drawings are Costly. — A drawing may be 
pleasing to the eye, but if the dimensions are inaccurate or the 
drawing is misleading it is useless for practical purposes. A 
mistake on a drawing may be the cause of much loss both in 
money and time, especially in a factory where manufacturing 
is done on a large scale. For example, suppose four hundred 
machines of a kind are to be built. Patterns have been made 
from the drawings and sent to the foundry for castings, which 
are made as fast as the facilities in the foundry will allow. 
The whole order might be filled before any of the machines 
are assembled, and if there has been an error on the drawings, 
some of the pieces may not fit into their places and may re- 
quire the making over of one or more parts; this will delay 
the erecting and result in extra expense for producing new 
castings, machining them, fitting them to their places, etc., 
thus delaying work all along the line — all of which might 
have been avoided if a certain figure had not been wrong on 
the drawing, an arrow point placed at the wrong line, or some 
other error made. 

It is customary in most factories to build a sample machine 



176 MECHANICAL DRAWING 

and test it before the design is approved and manufacturing 
orders issued to the shop. This first machine not only shows 
possible defects and errors in the construction, but it may 
show how improvements can be made. Frequently a desir- 
able change will be thought of while building the sample 
machine that would not have been noticed on the drawing. 

While the drawing must be correct it should also be legible 
and neat. Under the head of legibility comes the clearness 
of the drawing or the conveying of the idea that is in the 
designer's mind. The placing of the views, sections and pro- 
jections and the arrangement of dimensions, notes, etc., has 
much to do with the legibility of the drawing. Clear lines 
neatly joined, well-formed figures and letters, well-arranged 
dimensions, notes, etc., are among the essentials to neatness. 

Checking Drawings. — After drawings have been completely 
finished and contain all the necessary dimensions, symbols, 
abbreviations, notes, etc., it is the general practice to check 
them. The object of checking is not only to locate any errors 
that may have been made in the dimensioning, but to discover 
defects of any kind which should be remedied. A competent 
checker may suggest changes in design or in the method of 
manufacture. The checking may be done by the chief drafts- 
man or by one or more experienced draftsmen who have been 
assigned to this work. In smaller drafting-rooms, the drafts- 
men often check each other's drawings, but a man should not 
check his own drawing, if this can be avoided, because he is 
not so likely to detect his own mistakes as someone else. 

The tracing is generally used when checking, although some 
prefer a blueprint from the tracing. When using a blueprint, 
all corrections or changes may be indicated on the print in 
red pencil, and all figures that are correct may be checked 
with, say, a yellow pencil. After all changes have been ap- 
proved, the changes indicated in red are made on the original 
prints which are afterward compared by the checker with 
the blueprint to see if all changes have been made correctly. 
This checked blueprint may also be filed away for reference 
purposes in case there is any doubt as to who is to blame. 



CHECKING DRAWINGS 177 

After a tracing has been checked, the initials of the man 
checking it and the date should be placed on a space provided. 
Checking Lists. — The checking of the drawings should be 
done in a systematic way, and checking lists are often issued 
which show just what requires checking or, at least, the essen- 
tial details of this work. These lists are prepared partly with 
reference to the product of the plant or the conditions peculiar 
to it, although many items found in checking lists apply re- 
gardless of the class of work. A typical checking list follows. 
In using this list, or a similar list, items should be checked in 
whatever order is given, to avoid missing any of them. 

1. Is the size of sheet correct? 

2. Are the title, scale, drawing number, model, number re- 
quired, etc., correctly given? 

3. Is there a sufficient number of views to show the piece 
correctly? 

4. Are full and dotted lines shown in their proper places? 

5. Are dimensions properly located? 

6. Are all required tolerances properly given? 

7. Are views shown correctly as to right and left hand? 
Are even numbers used for right-hand patterns and odd num- 
bers for left-hand? 

8. Is the design correct in principle? 

9. Is it what is needed and can nothing better be suggested? 
Can it be made cheaper? 

10. Is the drawing correct to scale, and are those dimen- 
sions not scaling correctly underlined? 

11. Are arrow-heads neatly and properly shown and have 
any been omitted? 

12. Is all necessary information given, and are all dimen- 
sions "tied up"? 

13. Are all dimensions given in decimals where required? 

14. Are tapped holes shown correctly? 

15. Are "/" marks shown where needed? 

16. Is the proper draft provided for all patterns and forg- 
ing dies? 

17. Are all corners provided with rounds or fillets? 



178 MECHANICAL DRAWING 

18. Is a note given in regard to counterboring, spot-facing 
or other finish for screw-heads? 

19. Are all bosses large enough? 

20. Are all parts of proper strength? 

21. Are detail notes provided regarding heat- treatment, 
polishing, electro-plating, etc.? Are such notes correct? 

22. Are parts marked " grind," where needed? 

23. Are all given dimensions correctly figured? 

24. Will the piece properly fit parts with which it is to be 
assembled, and will it work without interference? 

25. Is clearance provided for wrenches and screwdrivers? 

26. Is there clearance provided to allow for all variations 
and tolerances? 

27. Are proper oil-holes provided, and is there a sufficient 
number of them? 

28. Are all parts provided with a sufficient number of 
threads of proper pitch for the material used? 

29. Is the allowance for driving and running fits expressed 
in thousandths of an inch, and are the parts marked with their 
tolerances? 

30. Are developed lengths of parts shown? 

31. Are parts to be ground "necked" to provide clearance 
for the wheel when grinding? 

32. Is provision made on all drill jigs, fixtures, etc., for the 
removal of burrs and chips? 

S3. Are the name, material and pattern number correct for 
each piece? 

34. Has proper consideration been given to the subsequent 
attachment of other parts? 

35. Is the material cross-sectioned according to standards? 

36. Has the following information been given regarding 
springs: Temper, gage number and decimal diameter of wire, 
number of coils, initial tension, inside diameter and length in 
compression? 

37. Is it shown on dies where they are to be ground? 

38. Is clearance provided for the leaf swing on jigs and 
fixtures? 



CHECKING DRAWINGS 1 79 

39. Can the bosses shown be drawn from the sand? 

40. Are you willing to stand responsible for any errors 
noted above, if this drawing is sent into the factory? 

41. Have you signed this drawing as " checked"? 

Checking List for Punch and Die Drawings. — The check- 
ing list winch follows applies more especially to drawings for 
punch and die work. This list was compiled with the idea 
of making it brief, as it is intended to be used in conjunction 
with more complete instructions. The sole object of the list 
is to call instantly to the mind of the checker the requirements 
of the standard practice. This list is subject to more or less 
modification, according to the practice and working conditions 
of different drafting-rooms, and it is divided into five sections 
headed: " Design and Approval"; " Assembly"; "Details"; 
"Title"; and "General Requirements." 

Design Approval: 

1. Authorization, requisition, memorandum, blueprint, 

sketch or sample. 

2. Yearly requirements. 

3. Grade of tool. 

4. Method of operation. 

5. General and specific requirements of departments. 

6. Harmony in design, compared with other up-to-date 

tools. 

7. General design. 
Assembly: 

1. Views and projections. 

2. Work to be easily placed in die. 

3. Work to be easily removed from die. 

4. Same gaging points on succeeding operations. 

5. Parts to be readily machined and assembled. 

6. Interference of moving parts, slides, etc. 

7. Burr side of blank in proper relation. 

8. Provision for grinding. 

9. Stripping and knockout devices to be adequate. 

10. Clearance for slugs and burrs. 

11, Setting pins. 



180 MECHANICAL DRAWING 

12. Safety pins for unsymmetrical blanks. 

13. Punch height to clear work during forming operation. 

14. Size of punch shank to be standard. 

15. Relation of shut height to available presses. 

16. Size of dowel-pins and standard screws. 
Details: 

1. Views and projections. 

2. Views of details placed in same relative positions as 

assembly. 

3. Detail to check with assembly. 

4. Easily machined. 

5. Easily assembled. 

6. Easily hardened without liability to check. 

7. How fastened in place. 

8. Scaling and calculating of dimensions. 

9. Intermediate dimensions. 

10. Over-all dimensions. 

11. Limits. 

12. Size and location of holes. 

13. Finish marks. 

14. Grinding marks. 

15. Detail number. 

16. Name. 

17. Number of pieces required. 

18. Material. 

19. Hardened. 

20. Ground. 

21. Forging. 
Title: 

1. Class and type of punch and die. 

2. Part number. 

3. Model number. 

4. Scale. 

5. Initials. 

6. Drawing number. 

7. Date drawn. 

8. Date traced. 



CHECKING DRAWINGS 181 

General Requirements: 

i. Neatness and clearness. 

2. Crowding- of views, details and notes. 

3. Lettering. 

4. Lines. 

5. Section-lining. 

The different men in the drafting-room use all or part of 
this list, as required. The designer uses the section on "As- 
sembly." The head of the division or department uses the 
sections under the headings "Design Approval," and "Assem- 
bly." The detailer uses the portion under the headings "De- 
tails," "Title," and "General Requirements." The tracer 
need pay attention only to the section of "General Require- 
ments," after which the checker uses the complete list with 
the exception of "Design Approval." It is understood that 
before a list of this kind can be used successfully, detailed 
instructions in separate form must be given to the draftsmen, 
covering all the parts to which attention is called in this list. 

Standardization of Drawings. — A common cause of mis- 
takes and misunderstandings in manufacturing is the lack of 
uniformity of practice in making drawings. Difficulty from 
this source is often experienced in jobbing shops which bid 
on contracts after having studied the drawings submitted. 
Frequently these drawings leave much to inference and im- 
agination, but it is a serious matter to bid on a product in 
the belief that a certain standard of manufacture is required 
when a higher or a lower one is wanted. If the contractor 
assumes that the standard is too high, he probably bids too 
high, and if he assumes that it does not call for high-grade 
work, his bid may be too low for making a living profit. 

The work of standardization should include fixing conven- 
tions, the adoption of uniform methods of indicating limits 
and tolerances, the placing of dimensions, the uses of dotted 
and broken lines, etc. — in short everything which makes a 
drawing a conveyor of specific instructions for making a part. 



CHAPTER VIII 

PRINTING PROCESSES AND APPARATUS FOR PRINTING, 
WASHING AND DRYING 

The original tracings of mechanical drawings are not in- 
tended for shop use, because they are a permanent record 
and would soon be soiled and perhaps lost if sent to the pat- 
tern and machine shops. The general method of securing 
reproductions of tracings suitable for shop use is by a process 
similar to that of the photographic printing process. There 
are several different kinds of prints, but the most common is 
known as a "blueprint" because it has white lines on a blue 
background. There are also prints which have white lines 
on a dark brown background, and also positive prints with 
either black or blue lines on a white background. These vari- 
ous prints are made on different kinds of paper which have 
been prepared for the purpose, and which may be obtained 
from any concern dealing in drafting-room supplies. 

Making Blueprints. — The process of making blueprints is 
simpler than making prints from photographic negatives, 
because the blueprint paper, after exposure, is simply im- 
mersed in water and chemical solutions are not required. 
When a blueprint is to be made, the tracing is placed in a 
printing frame which may be quite similar to the printing 
frames used in photography, except that it is usually much 
larger. One of these printing frames is illustrated in Fig. i. 
The frame is provided with a plate of glass and a removable 
back formed of sections that are hinged together so that one 
or more sections may be swung back for examining the blue- 
print, if necessary. Beneath the back of the frame there is a 
thick pad which serves to hold the tracing and blueprint flat 
against the glass. The back, in turn, is held firmly in posi- 
tion by cross-bars provided with flat springs. In the illus- 

182 



PRINTING, WASHING AND DRYING 183 

tration, one of these bars is shown removed and that section 
of the frame at the extreme right is turned upward. 

The tracing is first placed in the frame with the inked-in 
side next to the glass. Then the blueprint paper is inserted 
with the sensitized face next to the tracing. After the frame 
is closed and locked, the tracing and the sensitized paper back 
of it are exposed to the light for a length of time depending 
upon the kind of paper used. After exposure, the sensitized 
paper is removed and immersed in water. Almost imme- 
diately, white lines appear corresponding to the black lines 




Fig. 1. One Type of Printing Frame used for making Blueprints 

on the tracing. This is due to the fact that the exposure to 
light has a certain effect on the chemicals which form the 
coating of the blueprint paper. The result is that those parts 
of the paper which have been protected from the light by the 
black lines turn white when the paper is washed in water, and 
the exposed part changes to some shade of blue. If the ex- 
posure has not been long enough, a light shade of blue is ob- 
tained and, if it is too light, the blueprint will not be clear 
and distinct, because there is not enough contrast between 
the background and the white lines forming the drawing. On 
the other hand, if the exposure has been prolonged too much, 
the blueprint may be entirely spoiled. 



1 84 MECHANICAL DRAWING 

Printing frames of the general type used for solar or sun 
printing are often provided with a wheeled carriage, the frame 
being held in position by trunnions so that it can readily be 
turned over and be held at any angle which will give the best 
light during exposure. Many drafting-rooms which rely on 
solar printing have one or more printing frames which, with 
the carriage, are mounted on a track so that the frames can 
easily be pushed out through a window in order to secure 
better light. Whenever possible, the printing frame should 
be exposed to the direct action of the sunlight. 

Kinds of Blueprint Paper. — Blueprint paper, before ex- 
posure, has a yellow-green color, and if it is exposed to the 
light, but not immersed in water, the yellow-green shade 
changes to a grayish-blue. The blueprint paper before using 
should be kept in a tube or in some form of receptacle which 
excludes the light. Before making a blueprint, small sample 
prints are sometimes made to determine the time of exposure, 
especially if a new paper is being used. Blueprint papers 
vary greatly in regard to their sensitiveness. For instance, 
some papers require an exposure of only one minute or less 
and others, from four to eight minutes' exposure in bright 
sunlight. The papers requiring a longer exposure are the 
most satisfactory in regard to quality and the appearance of 
the prints. The sensitive rapid-acting papers are intended 
for use where prints are required quickly or where a strong 
light is not available and a sensitive paper is necessary. These 
" quick" or " rapid" papers need to be handled more care- 
fully than those that are less sensitive in regard to protection 
from light and dampness before exposure. Another kind of 
blueprint paper which is very sensitive is the kind used for 
printing with an electric light, as will be described later. 
Blueprint paper comes in rolls which may vary in width from, 
say, 24 to 54 inches and in length from 10 to 50 yards. Blue- 
print cloth may also be obtained. These cloth prints will 
withstand rough handling and are especially desirable for use 
out of doors or wherever ordinary paper prints would be easily 
torn. 



PRINTING, WASHING AND DRYING 



185 



Blueprinting Machines. — Machines designed especially for 
blueprinting are now in common use, especially in the larger 
drafting-rooms where a great many blueprints are being made 
constantly. With these machines, blueprints can be made at 
any time of the day or night, as a strong electric light is used 
for printing instead of 
the natural light or sun- 
light. Since these ma- 
chines are not dependent 
upon weather conditions 
and enable prints to be 
made rapidly at any time, 
they have a decided ad- 
vantage over the solar 
printing method. These 
machines are made in 
vertical and horizontal 
types which vary in re- 
gard to general arrange- 
ment and constructional 
details. The machine is 
provided with some 
method of adjustment 
for adapting it to the 
sensitiveness of the blue- 
printing paper used, and 
if desired the printing 
can be continuous, the 
roll of blueprint paper 
being fed right through 
the machine. Some ma- 
chines are equipped with apparatus for washing and drying 
the prints, but ordinarily the washing and drying is done 
separately and in the same way as when printing by means 
of sunlight. 

Vertical Type of Electric Blueprinting Machine. — A blue- 
printing machine of the vertical cylindrical type is illustrated 




Fig. 2. 



Vertical Type of Electric Blueprinting 
Machine 



1 86 MECHANICAL DRAWING 

in Fig. 2. This machine has two half cylinders of polished 
plate glass which are held in position by a suitable frame. 
Each half of this cylinder is provided with a curtain to hold 
the tracings and whatever paper is being used, against the 
glass during the time of exposure. The light for printing is 
obtained from an arc lamp which may be seen in the machine. 
This lamp is supported by a projecting arm and travels down 
through the center of the glass cylinder when making an 
exposure. The length of the exposure depends upon the 
speed at which the light travels, which is regulated by a suit- 
able governing device. As the tracing paper must be firmly 
and evenly held against the surface of the glass cylinder, the 
curtains are held at a constant tension, regardless of the posi- 
tion of their rollers, by weights attached to small wire cables 
which engage both ends of the rollers. There are two of these 
rollers, one on each side of the machine. Each operates inde- 
pendently so that one side only may be used, or one side may 
be unloaded and reloaded while the other side is printing. 
If desired, the machine may be loaded with a number of small 
prints at one time. This particular machine is the product 
of the Buckeye Engine Co., Salem, Ohio. The oil governor 
which regulates the speed at which the arc lamp travels down 
through the cylinder consists of a pump which is driven by a 
drum around which the lamp cable is wrapped. This pump 
forces oil through an orifice, the size of which may be regulated 
for varying the speed. There is also an automatic cut-out 
by means of which the current is automatically shut off when 
the lamp reaches the bottom of the cylinder. 

Flat-glass Type of Electric Blueprinter. — The flat-glass 
type of electric blueprinting machine shown in Fig. 3 has a 
curtain which is in the form of a continuous belt. This can- 
vas belt is mounted on a series of flat ribs. It is held against 
the flat glass by means of adjustable pressure shoes, and sup- 
ported by flanged sprocket wheels which assure uniform travel 
of the curtain. A motor with a fan attachment actuates the 
curtain and cools the machine. While standing at the front 
of the machine, the operator can change the curtain speed 



PRINTING, WASHING AND DRYING 



l8 7 



from zero to the maximum and he can also stop or start the 
curtain while the motor is running. Rolls of blueprint paper 
and tracings are supported by a trough. The paper can be 
cut to any size while the machine is feeding, if so desired, a 
cutting edge being provided for that purpose. After passing 
through the machine, the blueprint and tracings are delivered 
to a receiving tray. There is also an automatic receiving 
roller which will take up complete rolls of printed paper or any 




Fig. 3. Flat-glass Type of Electric Blueprinter 



desired part of a roll. The arc lamps, rheostats, and wiring are 
within the cabinet of the machine. Arc control switches regu- 
late the resistance of the rheostats, which regulate the size 
and intensity of the arcs. 

Paper for Electric Blueprinting. — The essential thing in 
electric blueprinting is the paper. Sun paper cannot be used 
profitably for electric printing, so a special " extra rapid electric 
paper" is made for the use of artificial light. A paper that 



1 88 MECHANICAL DRAWING 

will print in about thirty seconds in bright summer sunlight, 
and which has good keeping qualities, will prove satisfactory. 

Fresh paper prints more slowly than old, but washes more 
quickly; this means that the old paper prints more rapidly, 
but must be left in the bath for a longer period. The fresher 
the paper, the whiter the lines, while lines on old paper have 
a dull or grayish cast. In trying out new samples of paper, 
use small pieces along with one from the roll with which com- 
parison is to be made, and place all of them over one tracing. 

Method of Hanging Blueprints after Washing. — After 
blueprints have been washed, they may be dried by simply 
hanging them on a line or on racks so that the water will 
drain off by gravity. When blueprints are dried in this way 
they are often hung up so that the lower edge is in a horizontal 
position. This method is not a good one, because when a 
print is hung so that the lower edge is practically level, the 
water will gravitate to this edge and hang there in globules, 
and the print will not be perfectly dry at this edge for hours; 
and in almost every case the edge is discolored. The best 
way to hang blueprints for drying is to place them so that 
the lower edge is at an oblique angle with the horizontal; 
then the water will gravitate to the lowest corner. There 
will be no accumulation of water at other points, and the 
drying will be far more rapid than in the case when the sheet 
is hung level. This is a very small detail of drafting-room 
work, but it is of considerable consequence, both as regards 
time and the appearance of the blueprints. 

Blueprint Drying Rack. — The special drying rack illus- 
trated in Fig. 4 is made of wood and has about thirty-six slots, 
of the shape shown, cut in its under side; one side of each 
slot is vertical and one inclined at an angle of about 60 degrees 
to the horizontal, the two sides being connected by a circular 
arc. Each slot contains a small hardwood ball, which natu- 
rally tends to fall to the bottom or mouth of the slot. The 
width of this part of the slot, however, is less than the diam- 
eter of the ball, so that the latter cannot drop out. Hence, 
instead of dropping out, the ball exerts a pressure on each 



PRINTING, WASHING AND DRYING 



189 



side of the slot so that when the end of a blueprint is placed 
in position against the vertical face and the ball is allowed to 
fall to the bottom of the slot, the pressure exerted on the 
vertical face is quite sufficient to hold the print in place. To 
prevent the balls from falling out of the slots sideways, wires 
are secured to the rack opposite the middle of each slot, as 
shown in the illustration. 

The two principal advantages possessed by this piece of 
apparatus are: (1) The facility with which the ball is flipped 



O^ 




Machinery 



Fig. 4. Rack for holding Blueprints while drying 

up out of the bottom of the slot to release the blueprint. 
(2) The fact that the water is allowed to drain off the print 
without defacing it in any way whatever. This rack is in- 
tended for small and medium sized prints, but it can also be 
employed in drying large blueprints, though in this case it 
would be advisable to use two racks and suspend the prints 
from two points, in order to keep them as flat as possible. In 
this case, the two racks should be placed on an adjustable 
frame so that different sizes of prints could be accommodated. 
One of the racks could be fixed in position and the other made 
movable, one being arranged so that it is a little higher than 
the other. This precaution allows the prints to be so ar- 



190 MECHANICAL DRAWING 

ranged in the racks that the bottom edges of the prints are 
.not quite horizontal; the water on the prints will then flow 
to one corner and so drain away more readily than if the 
edges were horizontal. 

Drying Blueprints Rapidly. — Blueprints are often required 
without delay, and special apparatus has been made for dry- 
ing them rapidly after washing. A simple type of drying 
apparatus consists of an A-shape frame having a sheet-iron 
covering on one side with a trough at the lower end for catch- 
ing water which runs off from the prints. A print to be dried 
is placed against this sheet-iron tray and the water is removed 
from it quite rapidly by scraping the surface of the print 
with a rubber squeegee similar to the kind used for cleaning 
windows. This drying frame may be provided with some 
means of heating the interior. 

Blueprint Drying and Ironing Machine. — In large plants 
where thousands of blueprints are made, special apparatus is 
used which would be too expensive for smaller drafting-rooms. 
The blueprint ironing machine shown in Fig. 5 is an example 
of this special equipment. This machine not only dries the 
blueprints rapidly, but also irons them smooth so that they 
occupy less space when filed away in drawers or elsewhere. 
This machine has a large cylindrical cast-iron roll which is 
heated either by gas or steam. A fabric belt or apron runs 
over this roll. The blueprints to be ironed and dried are 
placed on this fabric belt, which carries them underneath the 
heated roll that moves at the same speed as the belt. After 
the print is passed under the roll, it is separated from the 
roll by means of a curved metal guard. The blueprint then 
falls back on the belt or apron and is carried to the opposite 
side of the machine. 

Washing, Drying, and Ironing Machine. — A still further 
development in the way of an apparatus for handling large 
numbers of blueprints rapidly consists of a machine for wash- 
ing, drying, and ironing prints after they leave the printing 
machine or frame. The exposed prints are placed upon a 
traveling belt which carries them under a series of pipes that 



PRINTING, WASHING AND DRYING 191 

spray water upon them. In this way, the prints are washed 
and they are then ready to be dried. The wash water is 
delivered by a series of pipes which extend the entire length 
of the belt. These pipes are equipped with spray nozzles 
which deliver water over the entire width of the belt, and 
after flowing over the blueprint, the water runs down into 
the tank, from which it is pumped back to the pipes. Before 
leaving the wet belt, fresh water is sprayed on the blueprint. 




Fig. 5. Blueprint Drying and Ironing Machine 

After being washed according to the method described in 
the preceding paragraph, the blueprint is taken up by a dry 
belt, which carries it over a stationary drier where the pre- 
liminary drying takes place. The print is then transferred to 
a heavy canvas belt which holds it in contact with a revolving 
drum that is heated by gas or electricity, as desired. The 
print remains in contact with this drum during one complete 
revolution and is then passed out of the machine to a table 
or stand placed in position to receive it. The contact with the 
drum has dried the print, and also "ironed" it, so that it 
emerges from the machine smooth. The only manual labor 



192 MECHANICAL DRAWING 

connected with the operation of this machine is to place the 
print in position on the first belt; after this has been done, 
the operation is entirely automatic. Prints may be finished at 
the rate of from 4! to 7! square feet per minute, and in ten 
hours, a 4 2 -inch machine will wash, dry, and iron 7000 square 
feet of blueprints. 

Restoring Over-exposed Blueprints. — When blueprints 
have been over-exposed, they may be restored unless the 
exposure was greatly prolonged. In many cases, it is simply 
necessary to leave the print in the water much longer than 
would otherwise be necessary. Incidentally, a print that has 
not been over-exposed should preferably remain in the water 
from two to five minutes in order to fix the blue background 
and prevent it from fading. While all prints fade somewhat 
when subjected to the direct sunlight, those that are poorly 
washed fade greatly. Sometimes hot water is used to restore 
an over-exposed blueprint, although a print immersed in hot 
water is distorted more than one that is washed in cold water. 
This, however, may not interfere with the usefulness of the 
print, as the dimensions required should be marked on it 
and not be obtained by measuring the print itself. 

A developer for restoring over-exposed blueprints consists 
of one-half ounce of saturate solution of bichromate of potash 
to each gallon of water in the bath. To use the potash eco- 
nomically, the bath water need not be changed every day, 
for by adding about two ounces of formaldehyde to a barrel 
of water as a deodorant, the water in the bath may be used 
for a week or two, even in very warm weather, without any 
odor arising. This germicide also destroys all sliminess in 
the pan, and even at full strength it does not injure the prints. 
It is, however, a powerful astringent, and if too much is used 
it will be harsh on the hands. Wet prints should never be 
hung in the light near a window, for they fade more quickly 
when wet than when dry. 

A simple method of restoring an over-exposed print is as 
follows: Remove the over-exposed print from the washing 
tank and while still wet lay it face upward on the table; then 



PRINTING, WASHING AND DRYING 1 93 

place an unexposed dry piece of blueprint paper of the same 
size over the wet one, and rub it with a piece of cloth or with 
the hands. This brings the two surfaces into intimate con- 
tact, and when separated it will be found that the over-exposed 
print is clear and of a rich blue color. There is one objection 
to this method, which is that a piece of blueprint paper is 
wasted for each print made. 

Making Blueprints from Blueprints. — A method of mak- 
ing blueprints from blueprints consists in coating the print 
with a little common kerosene and then printing immediately. 
The oil used will not affect the printing of the new print un- 
less it is applied very thickly, and it will not spoil the original, 
as it soon dries out, leaving it in as good condition as before. 
This process will also work perfectly when it is desired to make 
prints from heavy drawings. It has been used with success to 
make blueprints from printed matter. To make the original 
perfectly transparent, apply a coat of paraffine, boiling hot. 

Making Changes on Blueprints. — When slight changes are 
made on a tracing, the existing blueprints are frequently 
changed instead of making new ones, by using a special mark- 
ing fluid adapted for blueprints. A writing fluid for blue- 
prints may be made by adding enough washing soda to a 
small bottle of water to make a clear white line, and then 
adding enough gum arabic to prevent the solution from spread- 
ing and making ragged lines when applied to the blueprint. 
A suitable fluid may also be made by dissolving a crystal of 
oxalate of potash, about the size of a pea, in an ink bottle full 
of water. If the solution tends to run when applied to the 
blueprint, thicken slightly with mucilage. 

Whenever changes are made on blueprints, care must be 
taken that every print of that particular tracing is changed, 
preferably by checking up the list of prints if one is available. 
This practice of changing blueprints is not to be recommended, 
the better plan, as a general rule, being to make new prints 
after changing the original tracings. 

Mounting Blueprints. — In many shops blueprints are 
mounted in some way. The unmounted prints soon become 



194 MECHANICAL DRAWING 

soiled, they are easily torn, and loose prints are more likely 
to get lost. For mounting a blueprint, any good grade of 
cardboard, or a piece of sheet fiber about -^ inch in thickness 
and a little larger than the print, will be found satisfactory. 
The first step in mounting the print is to give both sides of 
the sheet of fiber or cardboard a coat of orange shellac and 
then set it up on edge to dry. After the shellac is thoroughly 
dry, a second coat should be applied to the side on which the 
blueprint is to be mounted. Then apply a similar coat of 
shellac to the back of the blueprint and allow the blueprint 
and mount to stand until the shellac is nearly dry, i.e., until 
the surfaces are so sticky that when touched with the finger 
tip, the mount can almost be lifted from the table. The 
blueprint should then be placed upon the mount and 
thoroughly rubbed down first with the hands and then with 
the roller. After allowing the mounted print to stand for 
four or five hours to allow the shellac to set thoroughly one 
or two coats of white shellac should be applied over the entire 
surface of the blueprint to give it a glossy appearance and to 
protect it from grease and dirt. The prints must be 
thoroughly dry before applying the white shellac. 

Mounting Blueprints on Cloth. — Blueprint cloth is often 
used in preference to paper when prints are to be used out-of- 
doors or are subject to rough handling. These prints are made 
directly on the specially prepared cloth. If ordinary paper 
blueprints are to be mounted upon cloth, the cloth should 
be soaked in water and then thoroughly wrung. The cloth 
should then be unfolded, shaken out, fastened to a drawing- 
board, and covered with flour or starch paste. This should 
be well rubbed into the cloth. The superfluous paste should 
be worked to the center of the board and then scraped off 
with the hand and returned to the basin. This process should 
be repeated until the paste is evenly spread all over the cloth. 
The blueprint should then be dampened on the back with a 
sponge, folded once, and placed in the correct position on the 
cloth. The rest of the work should be done by two persons. 
While the blueprint is held by the two corners at an angle 



PRINTING, WASHING AND DRYING 1 95 

of about 30 degrees, it should be rubbed gently but firmly 
with a large blotter, until one half is fastened to the cloth; 
the other half is tjien treated in the same manner. Any air 
bubbles that may appear can be pricked with a needle 
and the blotting pad pressed over it, while with a circular 
sweep of the other hand the mount is pressed firmly to the 
board. 

Blueprints from Typewritten Copy. — It is sometimes re- 
quired to make a typewritten copy from which a number of 
blueprints may be made, but if one attempts to make such 
blueprints from an ordinary typewritten sheet, the result is 
usually unsatisfactory because the purple or blue ink com- 
monly used on typewriter ribbons offers very little resistance 
to the passage of light. The result is that the blueprint has 
a weak and "washed-out" appearance. If, however, the 
stenographer takes two pieces of black carbon paper and 
places them face to face with a piece of thin white paper 
between, and then proceeds to write the copy in the ordinary 
way, it will be found that the letters, which are black and 
written on both sides of the sheet, are practically impervious 
to the passage of light. From a copy prepared in this man- 
ner, good clean-cut blueprints can be made. 

Blue- and Brown-line Prints. — Among the special prints 
which are sometimes used may be mentioned the Vandyke 
negatives, which have white lines on a brown background, 
and also the blue-line and brown-line prints. If positive 
prints are desired or those having either blue or brown lines 
on a white background, they may be obtained in the follow- 
ing manner: The original tracing is first used to make a nega- 
tive copy on thin Vandyke paper. This copy will have white 
transparent lines on an opaque dark brown background. 
Another print is then made by using this Vandyke negative 
in place of the original tracing, and the result is a positive 
print which will have dark lines on a white background if 
Vandyke paper is used, or blue lines on a white background 
if regular blueprint paper is used. Similar results may also 
be obtained on cloth by using either prepared blueprint cloth 



196 MECHANICAL DRAWING 

or vandyke cloth, depending upon whether a blue-line or a 
brown-line print is desired. 

Making Temporary Forms from Vandyke Negatives. — • 
When a small number of temporary forms are needed for 
special statements, inventory sheets, etc., and it would not 
pay to have the forms printed, the duplicate forms may be 
obtained by first making a tracing and then a vandyke nega- 
tive from which positive blue-line or brown-line prints are 
made. The brown-line prints are by far the more attractive 
and permanent, the lines showing much more clearly than 
the blue lines. 

The paper used as a base for sensitized brown print paper 
is a thin and very tough parchment, which admits of much 
handling without damage and is so transparent that, after 
having made skeleton prints of the forms desired, properly 
headed and ruled, if it is desired to reproduce the information 
after the form has been filled out, blue-line prints can be 
made directly from it. 

Photostats of Drawings. — A method of copying drawings, 
blueprints, etc., which is employed in some plants is to make 
copies by using an apparatus known as a "photostat." The 
photostat is a special camera having, in addition to a lens, a 
prism which prevents the part photographed from being re- 
versed, as in the case of a negative obtained with an ordinary 
camera. The reproduction obtained with a photostat is a 
negative, although positives can be obtained by a second 
photographing. If a photostat is made of a drawing formed 
of black ink lines on white paper, the reproduction will be 
reversed in color, the lines being light and the background 
dark. On the other hand, copies of blueprints have black 
lines on a white background, thus giving the effect of an orig- 
inal drawing. The photostat is sometimes used for copy- 
ing blueprints when the original drawing is not available, as, 
for example, when customers send blueprints to manufacturers, 
or when old prints of obsolete designs are needed for making 
repairs. 



CHAPTER IX 

ENGINEERING STANDARDS AND DRAWINGS OF 
MACHINE DETAILS 

A great many small parts or details are frequently used 
in the construction of mechanical devices, even when the 
design is not particularly complex, and it is necessary for the 
draftsman to know how to represent these details clearly and 
by methods which have been adopted to avoid useless and 
tedious work. For instance, screw threads are represented 
by certain common or " conventional " methods, as they are 
called, which make it possible to show a threaded section 
easily without attempting to draw a thread as it actually 
appears. The customary methods of drawing gears also illus- 
trate this feature of drafting practice. Thus, the teeth of a 
spur gear are commonly represented by dotted circles instead 
of attempting to draw the tooth outlines, as explained more 
fully later. In addition to these conventional methods, there 
are certain standards which have been adopted, and also com- 
mercial parts are used more or less in the construction of 
machinery and tools. It is necessary for the draftsman to 
distinguish between the parts that are made special and those 
parts which arc standard and perhaps purchased from other 
manufacturers. 

The standards which have been adopted for screw threads 
are the most important and the draftsman should be familiar 
with them. These screw thread standards vary in different 
countries and also for some special classes of work. They 
are so well established that all the principal standards are 
given in engineering handbooks. The tendency is to increase 
this work of standardization of machine details, because it 
results in greater economy in the production of machinery. 
Standardized machine parts can be made up and kept on 
hand for immediate use or, in some cases, they may be pur- 

197 



i g8 



MECHANICAL DRAWING 



chased in quantities from manufacturers equipped with special 
machinery adapted for the economical production of such 
parts. It is not only necessary for the draftsman to be fa- 
miliar with the different machine details and conventional 
methods in general use, but to understand how and where 
to employ them. 

Screw Thread Details. — As the screw and nut are used 
on almost every kind of mechanical device, the representation 
of screw threads is necessary on practically all mechanical 
drawings. Various forms of threads have been developed 
and some of these forms have become so generally used in 



ANGLE OF THREAD 



DEPTH OF THREAD 




Machinery 



Fig. 1. Drawing showing Meaning of Various Terms which are applied to 
Screw Threads 



different countries or for certain purposes that they are now 
termed "standard." The screw thread in most general use 
in the United States is the United States standard and if not 
definitely stated otherwise, threads shown on mechanical 
drawings are generally understood to be right-hand United 
States standard threads. 

Before considering the various conventional methods of 
representing screw threads, a few definitions relating to screw 
thread parts will be given. In Fig. i is shown a threaded 
portion of a bolt having a V-thread. A portion of the bolt 
is broken away at the right to show a sectional view of the 
thread, while at the left it is shown in full. 



MACHINE DETAILS 



IQ9 



Pitch and lead. — Pitch and lead are two expressions that 
are often confused. The word pitch is often, though errone- 
ously, used in place of " number of threads per inch." The 
pitch is the distance from center to center of two adjacent 
threads as indicated in Fig. 1. The lead of a screw thread is 
the distance the screw will travel through a nut if turned 
around one complete revolution. It is evident that for a 
single-threaded screw the pitch and lead are equal. In a 
double-threaded screw, as indicated in Fig. 2, the lead equals 
twice the pitch and in a triple- threaded screw, three times 





■< I > 








^ 1— ^ 




— *■ 


T r" 5 








T p^ - 






A A \ A \ 


M 


A MA) A 


llulW'TllU ; 


[ 




L \\ W \ \ \W 


u\n 


X 




A\\\\n\\\\l ■ 


- - 


* 


mWWWVW 


\ \ xx/X4 


. H« 


1 


\ \\\\v\\\\\\ 

U \ \\ \ ' U '\ \\ \ \\ \ U 1 \ 


L ' 


' 


Y/Vi \\\W \I\<\ \ 
\ v\ V\ VV v\ V 


\,Wy_] 


L J 


DOUBLE THREAD 




TRIPLE THREAD 


MacMnery 



Fig. 2. Double and Triple Threads of the Square Form 



the pitch. The definitions given for pitch and lead should 
be strictly adhered to, as great confusion is often caused by 
improper interpretation of these terms. 

Designating Multiple Threads. — Confusion is caused by 
indefinite designation of multiple- threaded screws. Some- 
times the lead and the class of thread are given, thus: u \- 
inch lead, double," which means a screw with a double thread. 
When cutting such a thread the lathe is geared for two threads 
per inch, but each thread is cut only to a depth corresponding 
to four threads per inch, and after one thread groove is 
finished, the work is indexed 180 degrees before cutting the 
second thread groove. This same thread might be designated 
by giving both the lead and the pitch in order to avoid any 
misunderstanding, thus: "|-inch lead, i-inch pitch, double 



200 MECHANICAL DRAWING 

thread." It is also well to mention whether the thread is 
right hand or left hand, R. H. or L. H. being used ordinarily. 

Representing Screw Threads on Drawings. — In drawing 
screw threads of any kind, the exact form of the thread as it 
appears to the eye or a correct orthographical representation 
of the thread is seldom shown for the reason that it would 
require an undue expenditure of time to draw the helical 
curves that are required to show the true form of any thread. 
Many simple methods of representing screw threads have 
come into use from time to time. Unfortunately, however, 
no particular method or methods have been actually adopted 
as standard practice, and mechanical drawings differ some- 
what in regard to such features. 

All the methods used to represent screw threads will not be 
referred to, but rather the ones most commonly used. Each 
method has perhaps some feature which makes it particularly 
adapted for certain purposes. Nearly all of the conventional 
methods in common use, such as are employed to represent 
small screw threads, etc., are not intended to show the form 
or shape of the part from the pictorial point of view. For 
instance, in the case of a thread, an explanatory note is used 
ordinarily to indicate the kind of thread and its pitch. This 
method eliminates an endless amount of tiresome work that 
would otherwise be required. When parts are threaded with 
standard taps or dies, the size of the tap or die and the length 
of the required thread are usually all the information needed 
on the drawing in addition to the conventional method used 
to represent the thread. The tap or die determine the form 
and pitch of the thread and, when threads are cut in a lathe, 
the machinist uses gages for determining the shape or form 
of the thread, and he needs to know the diameter and pitch 
or the number of threads per inch so that the lathe can be 
properly geared. It is, therefore, evident that no particular 
advantage is derived from a drawing which represents the 
actual outline of the thread when suitable notes are provided, 
unless it is some special form of thread which can be cut only 
by making special tools. 



MACHINE DETAILS 



20I 



Drawing Square Threads. — At A, Fig. 3, is shown a true 
drawing of a square screw thread, which is given here simply 
to show the appearance of a screw thread represented in this 
manner. The draftsman seldom uses this method of repre- 
senting a screw thread on mechanical drawings; however, it 
is sometimes desirable to show a thread in this way on patent 
drawings, catalogues, etc., where the pictorial effect is im- 
portant. When this is required, the rules for drawing the 
helix given in Chapter XI should be referred to. 




Fig. 3. Different Methods of representing a Square Thread 

At B, Fig. 3, is illustrated a method of drawing square 
threads which in appearance closely approximates the exact 
method. This method is too elaborate, however, for ordinary 
purposes and the simpler method illustrated at C is more gen- 
erally used. When a thread of considerable length is to be 
shown, unnecessary work is avoided by drawing two or three 
threads at each end of the threaded section and simply indi- 
cating the intermediate portion of the thread by dotted lines 
representing the bottom or root of thread as shown at D. 

Simpler methods than those already shown are often used 
and, when square threads of very small diameter are to be 
indicated, it is perhaps preferable to represent them by one 
of the methods illustrated at C, D, E and" F, Fig. 4. Of course 



202 



MECHANICAL DRAWING 



an explanatory note must accompany the drawing giving all 
information as to pitch, lead, etc., as otherwise it would ordi- 
narily be supposed to represent U. S. S. thread, assuming that 
the drawing were made in the United States. 

Common Methods of Representing Screw Threads. — In 
Fig. 4 at A and B are shown two V-threads represented by 
straight lines which, as in the case of the square screw thread 
shown at B, Fig. 3, represent the thread approximately as it 




Fig. 4. (A and B) Right-hand and Left-hand Screw Threads. (C, D, E, and F) 
Conventional Methods of representing Screw Threads 



actually appears. This method of drawing a V- thread is not 
generally used in making mechanical drawings but is given 
here chiefly to show the difference between a right-hand and 
a left-hand thread. The thread represented at A is a right- 
hand thread and that at B is a left-hand thread. A screw 
having a right-hand thread as shown at A will advance into 
a threaded hole when turned in a clockwise direction, and a 
screw having a left-hand thread as shown at B will enter a 
threaded hole when turned in a counter-clockwise direction or in 
a direction opposite to that in which the hands of a clock turn. 



MACHINE DETAILS 203 

A much simpler method of representing screw threads than 
that shown at A and B, and perhaps the most commonly 
employed method, is that shown at C and D. In this case, C 
represents a right-hand thread and D represents a left-hand 
thread. Although this method of indicating right- and left- 
hand threads is technically correct, the drawing itself should 
not be depended upon to show the difference between right- 
and left-hand threads. When a left-hand thread is required, 
it should be so stated on the drawing. If no note is given 
specifying right- or left-hand threads, it is understood that a 
right-hand thread is required. The use of the method shown 
at C to represent right-hand threads, and that shown at D 
to represent left-hand threads, is not restricted to the V- 
thread but is generally employed to represent screw threads 
of all forms. The light lines are intended to represent the 
top or the extended portion of the thread, while the heavy 
lines are intended to* represent the bottom of the recess or 
groove between the raised portions of the thread. In using 
this method of representing screw threads the draftsman 
usually draws the light lines first, making the spaces between 
the lines as uniform as possible, usually by judging the dis- 
tance as the triangle is moved to each successive position. 
After the light lines are all drawn in, the heavy lines may be 
drawn midway between the light lines as shown. 

Mechanical drawings for ordinary purposes do not require 
accurate spacing of the lines which represent the threads, 
but continued practice in drawing threads usually develops 
the draftsman's skill to a point where it becomes second 
nature to space the lines quite accurately and with consider- 
able speed. The spacing ordinarily need not be in accord- 
ance with the actual pitch of the thread, and in indicating very 
small threads the spaces can be made equal to two or three 
times the actual pitch. Many draftsmen make a practice of 
drawing the lines which represent the threads, at a slight 
angle. It is also common practice to draw these lines at 
right angles to the axis of the screw. This practice is recom- 
mended because it enables the draftsman to use the T-square 



204 MECHANICAL DRAWING 

or T-square and an ordinary triangle to much better ad- 
vantage. When large threads are to be shown or an excep- 
tionally accurate drawing is required, the lines representing 
the threads may be spaced with dividers and faint pencil 
guide lines drawn to indicate the depth of the thread so that 
the heavy shade lines may be all drawn to uniform lengths. 
Instead of using heavy lines to represent the bottom of the 
thread, as shown at C and D, Fig. 4, it is the practice in some 
drafting-rooms to make all the lines of uniform width. 

The method of drawing screw threads shown at E perhaps 
requires less time and care to produce neat appearing screw 
threads, than any of the other methods described, due to the 
fact that the center line and the line representing one side of 
the screw form guide lines that determine the length of the 
heavy lines as shown. It is not necessary, however, to make 
these lines heavier. It is customary to draw the short lines 
on the right-hand side of vertical screws and on the lower 
side of screws shown in a horizontal position on the drawing, 
as it gives the appearance of shading. One of the simplest 
methods of representing screw threads is shown at F, Fig. 4. 
This method is recommended for very small threads. The 
lines can, of course, be drawn at right angles to the axis of the 
screw or be inclined as shown in the illustration. 

Representing Threads in Holes. — Some of the most com- 
mon methods of representing threads in holes are shown in 
Fig. 5. The upper and lower views show, respectively, the 
methods of representing threaded holes in plan and eleva- 
tion, or as seen from the top and side. At A is shown the 
simplest plan view of a threaded hole, and this method is 
generally used. The method shown at B is identical with 
that shown at A except that the conventional method of shad- 
ing holes is used to strengthen the pictorial effect. The 
method shown at C is also intended to give a shaded effect, 
but it is not generally employed. Other variations are shown 
at D and E. 

The side view A shows one of the most common methods 
of representing screw threads in concealed holes. It will be 



MACHINE DETAILS 



205 



noted that this method is similar to that shown at C, Fig. 4, 
to represent an external screw thread. Practically the only 
difference is that dotted lines are used instead of light and 
heavy full lines. Screw threads in holes are always repre- 
sented by dotted lines, except in the case of sectional draw- 
ings such as are shown at D and E, Fig. 5. At B is shown a 
method which many draftsmen consider good practice for 
screws of all diameters. In representing very small screws 
by this method, the draftsman frequently draws the dotted 




A 




Machinery 



Fig. 5. How Threads in Holes are represented on Drawings 

lines free-hand in order to save time. If large threads or par- 
ticularly accurate drawings are required, guide lines and the 
60-degree triangle can be used to represent the actual outline 
of the thread. The method shown at C is perhaps the 
simplest and easiest for representing threaded holes and is 
particularly recommended for threads of small diameter. 
The threads on small machine screws, etc., are frequently rep- 
resented in this way. At D is shown the conventional method 
of representing a sectional view of a hole having a right-hand 
thread. It will be observed that the lines representing the 
threads are inclined in a direction opposite to that of the hid- 
den lines shown at A. The reason for this is that the 



206 MECHANICAL DRAWING 

threads at the farther side of the hole only are visible in the 
sectional drawing. At E is shown another method which is 
sometimes preferred when very large threads are to be drawn. 
When no great accuracy is required, method E can be used 
to show small threads by drawing the lines free-hand, using 
perhaps faint guide lines to determine the top and bottom of 
the threads. When a thread of large diameter is represented 
by this method, its pictorial effect may be improved by draw- 
ing lines which represent the top and bottom of the thread 
at the back of the hole. The thread will then have an appear- 
ance similar to that of the screw shown at B, Fig. 4, and the 
sectional view of an internal left-hand thread will have an 
appearance similar to that of the right-hand screw thread 
shown at A, 

Screws and Bolts. — Screws and bolts may be divided into 
a number of classes, each of which is particularly adapted to 
a certain kind of work. A draftsman should not only be 
familiar with the different classes but should know how to 
employ them to advantage in designing machines. Screws 
and bolts have to a certain extent become standardized and, 
as various standards are employed in different shops, the 
draftsman should ordinarily specify such standards as are 
kept in stock. The different classes used in machine con- 
struction are known as machine screws, cap- screws, studs, 
set-screws, and bolts. These general classes include different 
forms or types which are distinguished by variations of the 
shape of the head, etc. The term machine screw is applied 
to various forms of small screws but is generally understood 
to mean a screw the threads of which enter a tapped hole in a 
machine part and one which is provided with a slotted head 
to receive a screwdriver. Machine screws of this class are 
generally designated by numbers instead of actual sizes, the 
numbers increasing with the diameter. 

Machine Screw Standards. — There is no universal stand- 
ard for machine screw threads, but the standard adopted by 
the American Society of Mechanical Engineers is the one 
most generally used in the United States. These machine 



MACHINE DETAILS 
Table 1. A. S. M. E. Standard Machine Screws 



207 



Num- 


Outside 


Threads 


Num- 


Outside 


Threads 


Num- 


Outside 


Threads 


ber 


Diameter 


Inch 


ber 


Diameter 


Inch 


ber 


Diameter 


Inch 


O 


0.060 


80 


7 


O.151 


36 


18 


O.294 


20 


I 


O.073 


72 


8 


0. 164 


36 


20 


O.320 


20 


2 


0.086 


64 


9 


O.177 


32 


22 


0.346 


18 


3 


O.099 


56 


10 


0. 190 


30 


24 


0.372 


16 


4 


0. 112 


48 


12 


0. 216 


28 


26 


0.398 


16 


5 


0.125 


44 


14 


0. 242 


24 


28 


O.424 


14 


6 


O.138 


40 


16 


0.268 


22 


30 


O.450 


14 



screws vary in size from 0.060 inch to 0.450 inch in diameter. 
The basic form of thread is the same as that of the United 
States standard system. The various sizes are given in Table 
1. Screws of this kind are often designated by the number 
and pitch. For example, a 14-24 screw as shown at A, Fig. 
6, would indicate a No. 14 screw having 24 threads per inch. 
Machine screws are classified according to the shape or form 
of the head. At A is shown a flat-head machine screw. This 



14-24 FLAT-HEAD 




Xai 




5-44ROUND-HEAD 



3-22 FLAT FILLISTER-HEAD 




4*3-56 OVAL FILLISTER-HEAD 




Machinery 



Fig. 6. Methods of designating Machine Screws on Drawings 



208 



MECHANICAL DRAWING 



form of screw is used when the head should be set either flush 
or slightly below the surface. The correct proportions of the 
head may be determined by the formulas accompanying Fig. 
7. It is not usually necessary to proportion correctly machine 
screw heads shown on working drawings. When it is neces- 
sary or desirable to show a correctly proportioned view of a 
machine screw head, the draftsman usually refers to a table 
giving each dimension, such as will be found in most hand- 
books. Machine screws similar to the flat-head type but 
having heads which are slightly oval, are sometimes used in 



Round-head Screws 



i D h 



A = Diameter of body 

B = 1 . 85 A - o . 005 = Diameter of head 

C =0.7 A = Height of head 

D = 0.173^4 +0.015 = Width of slot 

E = % C + 0.01 = Depth of slot 



Flat-head Screws 




V^7 ° 



T 



A == Diameter of body 

B = 2 A — o . 008 = Diameter of head 

A — o . 008 

C = = Thickness of head 

1-739 
D = 0.173^ +0.015 = Width of slot 
E = y 3 C = Depth of slot 



Fig. 7. Proportions of Round-head and Flat-head Screws 



place of the regular flat head where a finished appearance is 
required. The heads of these screws are countersunk so that 
the edge of the oval portion comes flush with the surface of 
the metal. The term " French head" is sometimes applied to 
this type of screw. 

Round-head machine screws, such as are shown at B, Fig. 
6, are used largely in connection with small finished work, 
and are particularly useful in the manufacture of sheet metal 
equipment. As will be seen, the heads of this type of screw 



MACHINE DETAILS 



209 



protrude beyond the work but, being rounded, are not objec- 
tionable and in some cases improve the general appearance 
or finish of the work. Fillister-head screws such as shown at 
C and D, Fig. 6, are used largely on machine work where a 
heavy screw or bolt is not required. The type shown at C is 
known as the flat fillister-head and is generally set in flush 
with the work or in some cases a little below the surface. 
The counterbored hole which receives the cylindrical head is 
usually a little larger than the head to allow for any eccen- 
tricity that may exist in the screw. Screws of this type are 



Flat Fillister-head Screws 




A = Diameter of body 
B =* 1 . 64 A — o . 009 = Diameter of head 
C = o . 66 A — o . 002 = Height of head 
D = o. 173 A + 0.015 = Width of slot 
E = y 2 C = Depth of slot 



Oval Fillister-head Screws 




A = Diameter of body 

B = 1 . 64 A -o . 009 = Diameter of head 

C = o . 66 A — o . 002 = Height of side 

D = 0.173 J. + 0.015 = Width of slot 

E = % F = Depth of slot 

F = o . 134 B + C = Height of head 



Fig. 7. Proportions of Flat Fillister- and Oval Fillister-head Screws 

sometimes used without countersinking the work, but this 
method is seldom used. The oval fillister-head D is some- 
times used in place of the type shown at C when a neat finish 
is required. The oval portion of the head projects above the 
surface of the work as indicated. Formulas for proportion- 
ing flat and oval fillister-heads are given in conjunction with 
Fig. 8. When a number of machine screws are equally spaced 
in circular formation, it is common practice to include a note 
stating the size and number of screws required. This may 
be written as shown at D, "4 #3-56 oval fillister-head,'' which 



2IO 



MECHANICAL DRAWING 



means that four equally spaced oval fillister-head screws are 
required, size No. 3 having 56 threads per inch. 

As tables are usually available which give the exact dimen- 
sions of different machine screw heads, the draftsman sel- 
dom finds it necessary to use the formulas accompanying 
Figs. 7 and 8, but as these types of screws are usually pro- . 
duced on automatic screw machines, it is sometimes necessary 
to make use of the formulas in designing tool equipment for 
producing the different forms. 

Cap-screws. — Cap-screws are employed for fastening two 
pieces together in much the same way as are machine screws, 
but they are made in larger sizes and are generally used for 






Machinery 



Fig. 9. Cap-screws 

heavier work. The types of cap-screws most commonly em- 
ployed have either square or hexagonal heads (see Fig. 9), 
which enables them to be tightened with a wrench instead of 
a screwdriver, although some cap-screws have either slotted 
fillister, button, or countersunk heads, the shapes being simi- 
lar to the machine screws shown in Figs. 7 and 8. The term 
" tap-bolt" is often applied to the hexagon- and square-head 
types of cap-screws. Cap-screws have not become so 
thoroughly standardized as have machine screws and bolts, 
and the dimensions of cap-screws made by different manu- 
facturers may vary to some extent. When giving the length 
of cap-screws, the thickness of the head is not included except 
in the case of countersunk-head cap-screws, which form has 
the thickness of the head included in the length. 

When drawing cap-screw heads, the draftsman usually 
refers to manufacturers' tables giving the required dimensions 



MACHINE DETAILS 



211 



if it is necessary that the drawing accurately represent cor- 
rectly proportioned screws. However, most draftsmen do 
not usually proportion the heads of screws or bolts from tables 
when making working drawings, as their familiarity with the 
various forms enables them to proportion these types with 
sufficient accuracy to meet all requirements of regular 
mechanical drawings. 

Machine Bolts, — Many types of bolts have come into use 
for various purposes and some of the most commonly em- 
ployed types have been standardized. The difference be- 
tween a bolt and a screw, according to the generally accepted 



HEXAGON-HEAD BOLT 
L 




COUPLING BOLT 




EXPANSION BOLT 



TAPERED BOLT 



Machinery 



Fig. 10. Various Forms of Bolts 

meaning of the term, is that nuts are used on bolts whereas 
screws are inserted into tapped holes; there are exceptions, 
however, to this general classification. When a bolt is used 
to connect two pieces, a hole is drilled through both pieces 
large enough to receive the body of the bolt, and the nut is 
screwed either directly against one part or against a washer 
placed between the nut and the work, thus clamping the two 
pieces firmly together between the bolt head and the nut. 
The hole through which the bolt passes may be slightly larger 
than the diameter of the bolt or the bolt may be accurately 
fitted in the hole to prevent lateral movement of the part 
instead of relying entirely upon the friction resulting from the 
pressure of the nut. 



212 MECHANICAL DRAWING 

Bolts are classified according to the shape of the head. 
Some of the principal forms are shown in Fig. 10. The hexag- 
onal and square types are used in machine construction. 
The button form is applied to a miscellaneous class of work, 
especially when it is desired to improve the finish where the 
bolt head protrudes. Carriage bolts, not shown in the illus- 
tration, have conically or spherically shaped heads similar to 
the button-head bolt, but in addition have a square section 
underneath the head to prevent the bolt from turning in the 
hole when tightening the nut. Bolts having countersunk 
heads which are slotted to receive a screwdriver are termed 
"stove bolts." The T-head bolt is extensively used for 
clamping castings and forgings to .various kinds of machine 
tools. The T-shaped heads of the bolts engage T- slots which 
extend along the table of the machine. After the bolt head 
is inserted in the T-slot it is given a quarter turn so that the 
projecting lugs forming the T hold the bolt in place. The 
hook-bolt is a special form having a hook-shaped head. It is 
used for clamping narrow pieces which would be excessively 
weakened by drilling. The eyebolt is so named because the 
head forms a loop or "eye" which may be used in different 
ways. They are used principally to provide a means of 
attaching a chain or hook to heavy machine tools, motors, 
etc. Expansion bolts are used for such purposes as attaching 
a pipe-hanger bracket or other part to a wall or ceiling of brick 
or concrete when a through bolt cannot be employed. Bolts 
of this type are made in quite a variety of designs. The 
nominal size represents the diameter of the bolt proper and 
not the diameter of the casing or expansion member. 

United States Standard Bolts. — United States standard 
bolts are the form most generally employed in the machine 
building industry of the United States. Table 2 gives all the 
principal dimensions of bolts and nuts from J inch to 6 inches 
in diameter. 

As United States standard bolt heads and nuts must be 
frequently represented on mechanical drawings, the draftsman 
should be able to draw these forms although ordinarily it is 



MACHINE DETAILS 



213 



Table 2. U. S. Standard Threads, Bolts and Nuts 

The tap drill diameters in the table provide for a slight clearance at the root of the thread, in order 
to facilitate tapping and to reduce tap breakages. 



Diameter 


No. of 
Threads 


Diameter 
at Root 


Diameter 
of 




Dimensions 


of Nuts and Bolt Heads 


Width 

across 

Flats for 

Square 

and 


Width of 

Hexagon 

across 

Corners 


Width of 
Square 
across 
Corners 


Thick- 


Thickness 




per Inch 


of Thread 


Tap Drill 


ness of 
Nut 


of Bolt 
Head 










Hexagon 










% 


20 


O.18S 


% 


1/ 


0.578 


O.707 


% 


X 


% 


18 


O.240 


% 


% 


O.686 


O.840 


% 


% 


% 


16 


O.294 


& 


% 


0-794 


O.972 


% 


% 


% 


U 


0-345 


% 


% 


O.902 


I. 105 


Vie 


% 


X 


13 


0.400 


% 


g 


I .Oil 


1-237 


% 


ft 


Ym 


12 


Q-454 


% 


% 


1 .119 


1.370 


9/ u 


% 


% 


II 


0507 


% 


iKg 


1 . 227 


1.502 


% 


% 


% 


IO 


0.620 


% 


IK 


1.444 


1.768 


% 


% 


\ 


9 


Q-73I 


% 


1K0 


1 .660 


2033 


% 


% 


1 


8 


0.838 


°°64 


I 5 ^ 


1.877 


2.298 


1 


% 


I V 8 


7 


O-930 


% 


1% 


2.C93 


2.563 


1% 


% 


l\ 


7 


1.064 


I%2 


2 


2.310 


2.828 


iK 


1 


1% 


6 


1-158 


* 1% 


2 3 /( 6 


2.527 


3-C93 


1% 


ift 


iM 


6 


1.283 


I ft 


2% 


2-743 


3-358 


1% 


i 3 / 6 


1% 


sK 


1.389 


1% 


2%6 


2.960 


3-623 


1% 


ift 


1% 


5 


1.490 


1% 


2% 


3.176 


3-889 


1% 


1% 


1% 


5 


1. 615 


1% 


2% 


3-393 


4-154 


1% 


1% 


2 


4)i 


1. 711 


1% 


3K 


3.609 


4.419 


2 


ift 


2% 


4% 


1 .961 


2}<J4 


3K 


4-043 


4-949 


2% 


1% 


2% 


4 


2-175 


2^4 


3^ 


4.476 


5-479 


2)i 


1% 


2% 


4 


2.425 


2% 


4M 


4.909 


6.010 


2% 


2% 


3 


3V2 


2.629 


2% 


4% 


5-342 


6.540 


3 


2ft 


3K 


3H 


2.879 


2% 


5 


5-775 


7.070 


3% 


2% 


3% 


3% 


3.100 


3% 


5 3 /s 


6.208 


7.600 


3% 


2% 


3% 


3 


3-317 


3% 


5% 


6.641 


8. 131 


3% 


2% 


4 


3 


3-S67 


3 5 /s 


6% 


7.074 


8.661 


4 


3X6 


M 


2% 


3-798 


3% 


6H 


75o8 


9-i9i 


4% 


3/4 


4 l A 


2% 


4.028 


4& 


6% 


7-941 


9.721 


4% 


3% 


4% 


2% 


4-2S5 


4 5 Xs 


7K 


8-374 


10. 252 


4% 


3% 


5 


2% 


4.480 


4%6 


7 5 ^ 


8.807 


10.782 


5 


3% 


S/4 


2% 


4-73o 


4% 


8 


9.240 


11. 312 


S% 


4 


s l A 


2% 


4-953 


5K2 


8% 


9-673 


1 1 . 842 


S% 


4 3 /e 


s% 


2% 


5-203 


5%2 


s% 


10. 106 


12.373 


5% 


4% 


6 


2% 


5-423 


5^ 


9% 


IO-539 


12.903 


6 


4^6 



not necessary to draw them accurately. In Fig. 11 is indi- 
cated the correct method of proportioning United States 
standard hexagonal bolt heads and nuts. With the diameter 
d given, the proportions of the head can be accurately deter- 



214 



MECHANICAL DRAWING 



mined by using the formulas given. First obtain the dis- 
tance across flats w (see drawing A) which equals ijd + | 
inch; now with this dimension as a diameter draw the circle 
b. Next complete the hexagon which forms the plan view of 
the head by drawing lines tangent to this circle employing the 
T-square and the 30-60-degree triangle. The side view of 
the bolt head can now be drawn. First project the lines which 
determine the position of the sides and corner of the head. 






«s c 


=» 

-Iff- 


1 


S S 




1 " 


1 




/ 







•i 



*e W 3> 

w 10 




■< 2 *" 


"^ 2 








t 


\ 






/ 


1 






<?> 


•*■ 





Machinery 



Fig. 11. Proportions of U. S. Standard Hexagon Bolts and Nuts 

Now draw the lines at right angles to the center line which 
determine the thickness of the head. Referring to the illus- 

tration it will be seen that the thickness is equal to -j which 

equals one hah the width across the flats. Now with a radius 

w 
equal to ^, strike the two arcs which complete the drawing. 

At B is shown a plan view of the same bolt head, but so located 
as to allow the side view to be drawn by projecting lines from 
the four corners. The correct distance across corners repre- 
sented by a is thus shown in this illustration. In making 



MACHINE DETAILS 



215 



mechanical drawings it is the usual practice to show this view 
rather than that shown at A. After drawing the two lines 
which determine the .thickness of the bolt head, the radius 
which is equal to the diameter d of the bolt is used to draw 
the arc required to complete the view of one face. With the 
radius r, which is usually obtained by trial, strike the two 
arcs which complete the two remaining faces. The nuts 
shown at the right of this illustration are drawn in a similar 




Fig. 12. Proportions of U. S. Standard Square-head Bolts and Nuts 



manner, the only difference being that the thickness is made 
equal to the diameter d of the bolt instead of one half the 
width across the flats. 

United States Standard Square-head Bolts. — In Fig. 12 
is shown the method of drawing square-head United States 
standard bolts. In drawing the plan view shown at A it is 
customary first to draw a circle having a diameter w equal 
to \\d + \ inch. Then use the T-square and triangle to 
complete the head. The proportions given at A and B indi- 
cate the method of proportioning the head and nut respectively. 

14 L 



2l6 



MECHANICAL DRAWING 



The plan view of the bolt shown at C is produced by using 
the 45-degree triangle, drawing the lines tangent to the circle 
in the position shown. The method of projecting and pro- 
portioning the head and the nut D is clearly shown. 

Rounded Hexagon Bolts. — Rounded head bolts such as 
shown in Fig. 13 are sometimes used on finished machine 
work. It should be noted that the nut in this form differs 
from the head. This difference is caused by the hole break- 
ing through into the rounded portion of the nut. However, 




Machinery 



Fig. 13. Bolts having Rounded Heads for Use on Some Classes of Finished Work 

the thickness of the nut is made equal to the diameter of the 
bolt, as indicated. All the radii marked r are found by trial. 
S. A. E. Standard Screws and Nuts. — The standard screw 
threads adopted by the Society of Automotive Engineers are 
extensively used in the manufacture of automobiles. The 
shape of the thread is the same as the U. S. standard, but 
the number of threads per inch is greater. The length of the 
threaded portion equals one and one-half times the body diam- 
eter. The clearance between the top of the bolt thread 
and the bottom of the thread in the nut is proportionately 
the same as that adopted by machine screw makers, the top 



MACHINE DETAILS 



217 



being between 0.002 and 0.003 inch large. The body diam- 
eter of the screw is 0.00 1 inch less than the nominal diameter, 
and should be made within such limits that it is not larger 
than the nominal diameter, nor smaller than 0.002 inch less 
than the nominal diameter. The proportions of the S. A. E. 
standard screws and nuts will be found in tabular form in 
engineering handbooks. It is assumed that when screws are 
to be used in soft materials, such as aluminum, brass, etc., 
the pitch of the threads will be made to conform to the U. S. 





SQUARE-HEAD FLAT-POINT 



LOW-HEAD FLAT-POINT 



HEADLESS ROUND-POINT 



<— 




<} 




HEADLESS CONICAL-POINT 



SQUARE-HEAD CUP-POINT 



HANGER-POINT 



mQ mB 




ROUND-PIVOT 



SOCKET FLAT-POINT 

Machinery 



Fig. 14. Different Classes of Set-screws 



standard, in order to provide for adequate proportions of 
the thread. The S. A. E. screw standard supplants the 
A. L. A. M. standard adopted by the Association of Licensed 
Automobile Manufacturers in April, 1906. 

Set-screws and Studs. — Set-screws are generally used 
for holding two parts in a fixed position relative to each other. 
The set-screw is screwed through one part and its point is set 
against or into a machined recess in the other part as, for 
example, when a set-screw passes through a tapped hole in 
the hub of a pulley and bears against a shaft which drives 
the pulley. Keys are preferable to set-screws for locking 



2l8 MECHANICAL DRAWING 

pulleys, gears, etc., to their shafts and for similar work, al- 
though set-screws may serve the purpose when not subject 
to heavy loads. They are also^employed in conjunction with 
keys to prevent longitudinal movement of pulleys upon shafts. 
Set-screws are used not only for locking parts together but 
also as a means of obtaining slight adjustments such as 
required either to eHminate unnecessary play by means of 
gibs or for adjusting locating points in jigs, fixtures, etc. 
The different forms of set-screws most commonly employed 
in machine work are shown in Fig. 14. It will be observed 
that these differ as to the shapes of the ends and heads. They 
are made in other combinations, those shown with heads being 
made in headless forms and vice versa. Set-screws may be 
obtained in various lengths to meet requirements. The types 
used for some purposes are made from steel and hardened all 
over, while some are provided with soft heads and hardened 
points. 

The square-head flat-point set-screw is commonly employed 
in chuck and fixture work to prevent movement. This type 
was originally used to hold small pulleys, gears, etc., in place 
on shafts; however, the danger caused by the projecting 
heads on moving parts has resulted in the more general use 
of headless set-screws for this class of work. The low-head 
flat-point type is often used in place of square-head set-screws 
when only a small amount of clearance for the head is avail- 
able. The headless round-point set-screw is used very largely 
with jig or fixture work as an adjustable stop. Check-nuts 
are quite frequently used on headless set-screws to prevent 
any change in location, the same as on the other types. When 
provided with conical or cup points this type is frequently 
used in a collar to prevent its rotation on a shaft. When a 
conical-point set-screw is used for this purpose, the shaft is 
countersunk to receive the point. The angle found by the 
conical point is generally 60 or 72 J degrees, although this 
angle varies to a considerable extent in the products of vari- 
ous manufacturers. The cup point requires no countersink- 
ing operation as it imbeds itself in the metal by the pressure 



MACHINE DETAILS 



219 



exerted when tightening the screw. The pivot-point type of 
set-screw, sometimes termed the dog-point, is often used when 
the end is liable to become upset or enlarged as the result of 
numerous applications of pressure. It is also used in place 
of a key to prevent rotation when a longitudinal movement 
of a gear or pulley along a splined shaft is required. The 
socket flat-point set-screw is used in place of the square- 
head flat-point type and it is rapidly replacing the other types, 
especially where a strong set-screw is required on moving 
parts. This type is also provided with different type points 





-< — c 


I — > 


d= SET-SCREW DIA. 












t 




r 


7" 




< t 
















11 


1 


illlll 


111 




>^- 


— W 


-id 








.mPImIII/ 




/ 


\ 








B 


Machinery 



Fig. 15. (A) Proportions of a Set-screw Head. (B) A Stud 



and can generally be used for any class of work requiring set- 
screws. 

In drawing set-screws of the square-head type, it is cus- 
tomary to show only one of the flat sides of the head in the 
side view, as indicated at A in Fig. 15. Although there is no 
standard rule for proportioning set-screw heads, the propor- 
tions indicated at A are quite generally employed by drafts- 
men in representing set-screws of the square-head type. 
When exact dimensions of the various types of set-screws are 
required, they must generally be obtained from the manufac- 
turers, catalogue, as the commercial forms of set-screws have 
not become thoroughly standardized. 

Studs. — Stud bolts have a thread at each end. They are 
extensively used to hold cylinder heads, steam chest covers 
and similar parts, in position. The threaded section t of the 



2 20 MECHANICAL DRAWING 

stud shown at B, Fig. 15, is usually screwed into the sta- 
tionary part to which the removable cylinder head or other 
part is attached. The stud differs from a cap-screw in that 
the nut is substituted for a solid head. Studs are generally 
considered preferable to cap-screws, especially on heavy ma- 
chinery in which the attached part must be frequently re- 
moved. The thread of the end t is usually made a little over 
size to make it fit tightly in the tapped hole. When set into 
cast iron, the length of the threaded end should be at least i\ 
times the diameter of the stud. The threaded portion n on 
which the nut is screwed is made of any length to meet re- 
quirements, and for some purposes it is made a sufficient 
length to enable two nuts to be used instead of one, thus pro- 
viding means of fastening and locking the piece in place. In 
drawing studs, the conventional method of representing screw 
threads is generally used. The dimensions required include 
the length over all, the lengths of the threaded portions / and 
n, and the diameter of the stud. 

Pipe Threads. — Wrought iron and steel pipes are almost 
universally employed for conveying steam, water, and other 
fluids, especially if the pressure is in excess of 100 pounds 
per square inch. Such pipes are made in three thicknesses, 
known as "standard," " extra strong," and "double extra 
strong." The size of iron and steel pipe is specified in terms 
of the nominal inside diameter, although the actual inside 
diameter of standard pipe is slightly greater than the nominal 
diameter specified, and the actual inside diameter of the 
double extra strong, considerably less. The outside diameters 
of standard, extra strong, and double extra strong pipe are 
the same, the increased thickness of wall merely decreasing 
the internal diameter. 

Threaded joints are not as suitable for piping subjected to 
extremely high pressure as are some of the many types of 
pipe joints especially designed for strength. They are, never- 
theless, extensively used in joining wrought iron and steel 
pipe. The standard thread system for wrought iron and steel 
pipe used in the United States is known as the "Briggs standard 



MACHINE DETAILS 



221 



pipe thread." This thread is made with an angle of 60 degrees 
and, according to the original standard, it is slightly rounded 
at the top and bottom so that the depth of the thread, instead 
of being equal to that of the sharp V-thread, is only four 
fifths of the pitch. Owing to the difficulty of producing a 
thread with rounded top and bottom, the pipe thread in com- 
mon use is made practically sharp at the bottom while the top 
Table 3. Briggs Standard Pipe Thread 



Diameter of Pipe 


No. of 

Threads 

per 

Inch 


Diameter 
at End 
of Pipe 


Length 

of Perfect 

Thread 


Total 

Length 

of Thread 


Total No. 


Total 

Distance 
Pipe 
Screws 
into 


Nom- 
inal 
Inside 


Actual 
Inside 


Actual 
Outside 


of Turns 

Pipe 

Screws 

into 














Fitting 


Fitting 


% 


O.270 


O.405 


27 


0-393 


O.19 


O.412 


5-i3 


0. 19 


% 


O.364 


O.540 


18 


O.522 


O.29 


O.624 


5 


22 


O.29 


% 


O.494 


O.675 


18 


O.656 


O.30 


O.634 


5 


40 


O.30 


% 


O.623 


O.840 


14 


O.815 


0.39 


O.818 


5 


46 


0.39 


% 


O.824 


I.050 


.14 


I.025 


O.40 


O.828 


5 


60 


O.40 


I 


I.048 


I-3I5 


nK 


I.283 


O.51 


I.030 


5 


87 


O.51 


iK 


I.380 


I.660 


ntf 


I.626 


0.54 


1 .060 


6 


21 


0.54 


1% 


I. 6lO 


I .900 


11M 


1.866 


0.55 


I.070 


6 


33 


0.55 


2 


2.067 


2-375 


n l A 


2-339 


O.58 


I .IOO 


6 


67 


O.58 


2% 


2.468 


2.875 


8 


2.819 


O.89 


1 .640 


7 


12 


O.89 


3 


3.067 


3-500 


8 


3-441 


0.95 


1 .700 


7 


60 


0.95 


3K 


3.548 


4.000 


8 


3-938 


I .OO 


1-750 


8 


00 


I .OO 


4 


4.026 


4.500 


8 


4-434 


I.05 


1 .800 


8 


40 


I.05 


4K 


4.508 


5.000 


8 


4-931 


I. IO 


I.850 


8 


80 


I . IO 


5 


5-045 


5-563 


8 


5 -490 


1. 16 


1 .910 


9 


28 


1. 16 


6 


6.065 


6.625 


8 


6.546 


1 . 26 


2.0IO 


10 


08 


1 .26 


7 


7.023 


7.625 


8 


7-540 


I.36 


2 .IIO 


10 


88 


1.36 


8 


7.982 


8.625 


8 


8-534 


I .46 


2.2IO 


11 


68 


1 .46 


9 


8-937 


9.625 


8 


9-527 


i-57 


2.320 


12 


56 


i-57 


10 


IO.OI9 


IO.750 


8 


10.645 


1.68 


2.43O 


13 


44 


1.68 



is slightly flattened, the flat being carried down so that it 
just touches what would be the rounded part of the correct 
form. As thus modified, the depth of thread equals 0.833 
times the pitch. The taper of the thread on the diameter 
equals ■£$ inch per inch, or f inch per foot. The principal 
dimensions and specifications for Briggs pipe threads are 
given in Table 3. 

Pipe Fittings. — Different sections of pipe are connected 
by "pip e fittings," such as elbows, tees, crosses, flanges, unions, 



222 



MECHANICAL DRAWING 



couplings, etc. The size of a fitting corresponds to the nomi- 
nal size of pipe for which it is intended. The smaller pipes 
are commonly joined by some form of screwed connection, 
such as a coupling or union, but for sizes above 6 inches, 
flanges which are held together by bolts are in common use. 
Several standards have been adopted at different periods for 
governing the diameter and thicknesses of flanges and the 
diameter of the bolt circle, as well as the size and number of 




Machinery 



Fig. 16. Examples of Pipe Drawing 



bolts. The most recent of these standards is known as the 
American standard, which became effective January i, 19 14. 
The draftsman engaged on work which requires piping should 
secure tables covering the sizes of different classes of fittings 
and other data required in connection with the installation 
and laying out of piping. 

Drawings of Piping. — Small sizes of piping are sometimes 
represented diagrammatically by a single heavy line, but the 
larger sizes are usually drawn as illustrated in Fig. 16, which 



MACHINE DETAILS 223 

shows two sections. The upper illustration represents pipes 
which are connected with a tank, and the lower illustration 
shows valves in connection with a pipe drawing. When an 
entire lay-out of piping is to be represented in a single view, 
the isometric form of drawing is often very convenient. The 
principle governing this method of drawing is explained in 
Chapter XIII. When the diameter of wrought iron or steel 
pipe is given on a drawing, this corresponds to the nominal 
inside diameter (see column i, Table 2), except in the case of 
pipes larger than 15 inches in diameter; the sizes of the latter 
are based on the outside diameter. 

Designating Keys on Drawings. — As keys are used on 
nearly all classes of machinery, the draftsman should be fa- 
miliar with different types and the purposes for which they 
are used. Keys are ordinarily inserted in the hubs of pulleys, 
gears, etc., to prevent rotation relative to the shaft. The 
sunk key is the most common type. This is of rectangular 
section and engages a groove or slot formed both in the shaft 
and hub of the gear or pulley. The so-called saddle key does 
not enter a slot in the shaft, but is curved on the under side 
and is slightly tapered on top so that when driven into place 
the shaft is gripped by the frictional resistance. The flat 
key is a rectangular shape which bears upon a flat surface 
formed on one side of the shaft. The draw or gib key is a 
sunk key which has a head by means of which it can be re- 
moved. The round tapered key is simply a taper pin which is 
driven into a hole that is partly in the shaft and partly in 
the hub ; this form is used for light work. The name feather 
or spline is applied to a key which is fixed to either a shaft or 
hub, as when a gear must be driven by a shaft, but at the 
same time be free to slide in a lengthwise direction. The 
Woodruff key is a section of a disk, the edge of the part which 
enters the shaft being circular. 

The method of designating keys on drawings depends upon 
the type of key. In addition to the width and thickness of a 
key of the flat or sunk type, the taper, if any, would be given 
as a certain amount per foot. The taper of sunk keys is 



2 24 MECHANICAL DRAWING 

usually about § or T 3 ¥ inch per foot. The proportions of sunk 
keys are not governed by fixed standards. For instance, the 
width of the key may equal one fourth of the shaft diameter, 
and the thickness, one sixth of the shaft diameter, or these 
proportions may be varied somewhat by different manufac-. 
turers. The different sizes of Woodruff keys are designated 
by numbers, and in some cases, by letters. The round-end 
feather keys of the Pratt & Whitney Co. are also designated 
by numbers which correspond to those used for the Wood- 
ruff keys. 

Drawings of Gearing. — Draftsmen should understand the 
principles governing the design and action of gearing, because 
gears of some kind are used on such a large variety of mechani- 
cal devices. The types of gearing in common use include spur 
gears, bevel gears, spiral gears, and worm-gears. The points 
to consider in the design of gearing are: The type of gear to 
use; the relative sizes or diameters of meshing gears to secure 
the required speed ratio ; the size or pitch of the teeth so that 
they will be strong enough to transmit the required power; 
and the shape or arrangement of the body of the gear itself. 

The type of gearing to use depends upon such factors as 
the relative locations of the shafts and the reduction or in- 
crease of speed required between the driving and the driven 
shafts. The gear diameters are, of course, determined with 
reference to the speed ratio necessary and the pitch of the 
teeth depends upon the strength needed to transmit the power 
without danger of breaking the teeth. The body of the gear 
may simply be in the form of a disk or there may be a hub and 
rim connected by either a web or by means of arms or spokes. 
It is evident that the design of gearing is too broad a subject 
to be covered in a book dealing primarily with drawing ; hence, 
the common methods of representing gearing on drawings will 
be explained and illustrated, but the subject of gear design 
and the proportioning of gears for withstanding stresses and 
transmitting a given amount of power will not be considered. 
The student of drafting practice, however, should study this 
subject separately, owing to the extensive use of gearing. 



MACHINE DETAILS 



225 



Methods of Drawing Spur Gears. — Working drawings of 
spur gears, as they are usually made, do not show the gear 
as it actually appears, because the making of such a drawing 
would require considerable time and it would not be any 
better for shop use than the simpler drawings which are em- 
ployed. The conventional methods of representing gears 
differ somewhat in various drafting-rooms, just as in the 




_ 



MncMnery 



Fig. 17. (A) Drawing of Gear with all the Tooth Outlines shown. (B) Simpler 
Method of representing Gear which serves all Practical Purposes 



case of screw threads, but these differences in practice pertain 
to minor details. Drawing A, Fig. 17, shows two spur gears 
in mesh, and in this case the curvature or outline of each tooth 
is represented. It is evident that to draw in all of these 
teeth, even though the tooth curves are only approximately 
correct, requires considerable time, and is also unnecessary 
on a working drawing. Therefore, the teeth are either omitted 
entirely on a working drawing, as shown at B, or sometimes 



226 MECHANICAL DRAWING 

a few teeth are drawn in at the point where one gear meshes 
with the other. It will be noted that the teeth are repre- 
sented by dotted circles. The diameter of the outer circle 
corresponds to the outside or blank diameter of the gear, and 
the inner circle, to the bottoms of the tooth spaces. The 
dot-and-dash circle between these dotted circles is known as 
the pitch circle. When the diameter of a spur gear is referred 
to, it is always understood to mean the diameter of the pitch 
circle, and it is the relation between the diameter of the pitch 
circles of two meshing gears which determines their relative 
speeds. When a drawing shows two gears in mesh, the pitch 
circles are always in contact. 

The outer circles are sometimes drawn solid instead of using 
dotted circles. For instance, the outlines of three or four 
teeth may be drawn to the approximate shape at the meshing 
point, and then the remainder of the outer circle be represented 
by a solid line. If some gears are located back of others, as 
in a compound train, and solid circles are dotted to represent 
the concealed parts of the gear rims, which may be an ad- 
vantage in showing the relative locations of the gears, espe- 
cially if it is not convenient to include an end view. It will 
be understood that Fig. 17 is intended merely to show the 
difference between an actual representation of gearing and a 
common or conventional method of illustrating gears on work- 
ing drawings. On a regular working drawing the sizes of the 
gears and the pitch of the teeth would, of course, be given 
together with other necessary dimensions and information; 
then a drawing of the kind illustrated at B is just as useful 
to the machinist who has to cut the gear as a true drawing 
which shows the curvature of each tooth. 

Working Drawing of Spur Gears. — Most gears are made 
either of cast iron or of steel. The cast-iron gear may have 
a solid web or a thin section between the hub and the rim, or 
it may have arms or spokes, the latter being used for the 
larger sizes. Many small gears are made of steel, especially 
if they are intended for hard service. The steel gears of 
larger sizes are usually made from forgings or, if quite 



MACHINE DETAILS 227 

small, they may be turned from bar stock. The term "gear 
blank" is applied to any gear before the teeth are cut. When 
making working drawings of gears, the draftsman considers 
these points. For instance, if the gear is to be cast and is to 
have spokes, dimensions will be needed by the patternmaker 
which would not be required on a simpler form of gear. 

The amount of work necessary in designing a gear varies 
somewhat according to the size and type of gear and is also 
affected by the class of service the gear is intended for. For 
instance, a small steel gear which is to transmit little power 
may simply be in the form of a disk, and calculations for de- 
termining the pitch of the teeth or the proportions of the gear 
body are unnecessary. On the contrary, if a large gear is 
required for severe duty, the designer must carefully con- 
sider the pitch and strength of the teeth as well as the pro- 
portions of the different sections, such as the arms in the 
case of a cast-iron gear. The student of drafting should 
understand clearly the relation between these problems in 
design and the making of the drawing itself. 

In order to illustrate the important points to consider when 
making working drawings of spur gears, a practical example 
will be considered. Suppose two spur gears of 6 diametral 
pitch are to be located on shafts which are 12 inches apart as 
measured between centers, and that the speed ratio of the 
shafts is 2 to 1. The problem is to determine the size of each 
gear and to make a working drawing which gives all the in- 
formation necessary for making the gears. 

As the center- to-center distance is 12 inches, the total diam- 
eters of both gears equal 2 X 12 = 24 inches. Now, as the 
diametral pitch is 6 (which equals the number of teeth for 
each inch of diameter) it follows that the total number of 
teeth in both gears equals 24 times the diametral pitch, or 
24 X 6 = 144. Since the speed ratio is 2 to 1, one gear must 
be twice as large in diameter as the other and have twice the 
number of teeth; therefore, in this case, one gear has 48 teeth 
and the other, 96 teeth (96 + 48 = 144). Their respective 
pitch diameters which are obtained by dividing the number 



228 



MECHANICAL DRAWING 



of teeth by the diametral pitch, equal 8 and 16 inches 
(48 -f- 6 = 8 and 96 -r- 6 = 16). The outside or blank diam- 
eter of a spur gear is obtained by adding 2 to the number 
of teeth and dividing the sum by the diametral pitch. In 
this example, the outside diameter of the large gear equals 

96 + 2 

2 — = ^'ZSS mcnes > an d the outside diameter of the small 

48 + 2 

gear equals — = = 8.333 inches. The whole depth of the 

6 

teeth is found by dividing 2.157 by the diametral pitch and 



96 TEETH 6 D.P. 



-w— 




Machinery 



Fig. 18. Working Drawing of a Spur-gear Drive 



equals, in this case, 0.3595 inch. This is the amount that 
the cutter is fed in radially when cutting teeth (assuming that 
the gear blank is turned to the correct diameter), and it should 
preferably be given on a working drawing. 

Designers often use simplified formulas for proportioning 
such parts of a gear as the hub, rim, and spokes in case the 
gear is of that type. Such formulas are found in some hand- 



MACHINE DETAILS 229 

books and also in works on machine design. For instance, in 
the case of the gear shown in Fig. 18, certain dimensions are 
obtained by dividing constants by the diametral pitch. These 
constants have been found by experience to give the right 
proportions, and the designer's calculations are thus greatly 
simplified. If an attempt were made to calculate the exact 
sizes according to the theories governing the proportioning of 
parts to withstand certain stresses, the calculations in many 
cases would be extremely complex and the final results would 
not be as satisfactory in many instances as are obtained by 
the simpler and more rapid methods. 

It is not necessary to give dimensions or information on a 
working drawing pertaining to the curvature of the gear teeth, 
because the exact form of the teeth is controlled by the gear- 
cutting process. Practically all gears used at the present 
time have teeth of the involute form, so that this is assumed 
to be the case and need not be specified on a working draw- 
ing unless a special tooth form is desired. It is advisable 
for the draftsman to understand in a general way at least, 
how tooth curves are originated; therefore, the methods of 
drawing involute curves and also the cycloidal curves 
which formerly were used for gear teeth are explained in 
Chapter XI. 

Many gears have what is known as the "stub tooth. " Such 
gears are used in automobile transmissions and in other forms 
of mechanism which are subjected to severe service and are 
required to have exceptionally strong teeth. According to 
the method introduced by the Fellows Gear Shaper Company, 
the stub gear tooth is based on two diametral pitches; thus, 
if the pitch of a gear on a working drawing is given as § pitch 
(also written 6-8 pitch), this means that the height of the 
tooth conforms to 8 diametral pitch whereas the thickness, 
number of teeth, and pitch diameter are based on 6 diametral 
pitch. It is evident, therefore, that the tooth is thicker in 
proportion to its height than a standard tooth. According 
to the system of R. D. Nuttall Co., the tooth dimensions 
are based upon the circular pitch, the addendum being equal 



23° 



MECHANICAL DRAWING 



to 0.250 times the circular pitch, and the dedendum equal 
to 0.300 times the circular pitch. 

Drawings of Bevel Gears. — Bevel gears are often repre- 
sented on working drawings as though they had plain or 
smooth conical surfaces instead of teeth (as illustrated at A, 
Fig. 19). This method is more likely to be employed when 
the bevel gears are shown on the drawing as a detail or part 
of a complete mechanism which is represented by the drawing. 
A working drawing, however, could also be made in this way, 
provided the necessary data for cutting the gears were given 




Fig. 19. Two Methods of representing Bevel Gearing 



in the form of notes. A common method of representing 
bevel gears on working drawings is illustrated at B. This is 
simply a sectional view, and, when properly dimensioned and 
accompanied by certain notes, it gives all the information 
necessary for making gears. 

As a practical example of bevel gear drawing, assume that 
two shafts at right angles to each other are to be connected 
by gearing of 3 diametral pitch having a ratio of 4 to 1 be- 
tween the driving pinion and the driven gear. The smaller 
gear (commonly called the pinion) is to have 15 teeth, and a 
working drawing is required for cutting the gears with a formed 



MACHINE DETAILS 



231 




151- 



232 MECHANICAL DRAWING 

milling cutter. As the ratio is 4 to 1, the large gear will have 
60 teeth. With these figures available, the required angles 
of the tooth faces, the diameters and other dimensions can 
be determined by the rules or formulas found in handbooks 
and in treatises on gearing. (See Machinery's Handbook.) 
Before the bevel gear blanks are turned, it is necessary 
to determine their diameters and the angles of the conical 
faces, and before the teeth can be cut, the angles at which 
the blanks must be held in the machine are required. In 
addition, various other dimensions and information are neces- 
sary on the working drawing, as illustrated in Fig. 20, which 
shows a drawing for the particular combination of gearing 
referred to. This drawing includes, in addition to a sectional 
view of the gear and pinion, a detail plan illustrating the shape 
of the arm and web, and also an end view of the pinion showing 
the cross driving slot which is used in this case instead of a 
key because the pinion is so small. 

Some of the information required in cutting the gear is given 
in tabular form in preference to placing the figures directly on 
the drawing. It will be noted that the face angle of 12^ de- 
grees and also the face angle of 76 degrees are given with 
reference to a line at right angles to the axis of the gear; con- 
sequently, these angles correspond to the angles at which 
the machinist would set a compound rest for turning these 
surfaces in a lathe. It will be noted that on each hub a cer- 
tain amount of stock is allowed for fitting, as indicated by the 
dotted line. No dimension is specified on this particular 
drawing, but instead the drawing is marked "make all alike." 
The object of making them all alike is to avoid resetting in 
the gear-cutting machine when cutting the teeth in a certain 
lot or number of gears. 

Drawings of Worm-gearing. — A detail drawing of a worm 
and worm-wheel is shown in Fig. 21. The worm is not shown 
in position, as an end view would then be obtained. These 
two sectional views with the accompanying notes give all the 
information necessary. The distance from the center of the 
worm-wheel to the center of the worm, when the latter is in 



MACHINE DETAILS 



233 



position, is given on the drawing of the worm-wheel, and in 
this case is 4.672 inches. The ends of the blank for the worm 
are turned down to a diameter of 1.8 13 inches, which corre- 
sponds to the root diameter of the thread and serves as a guide 
for depth when cutting or milling the thread. The pitch of 
the worm and the circular pitch of the worm-wheel are, of 




WHEEL 

NUMBER OF TEETH =45 

CIRCULAR PITCH = 0.500" 

ANGLE OF CUT=8°20 / 

WORM, DOUBLE, R. H. 

OUTSIDE DIAM. OF WORM = 2.500" 



Machinery 



Fig. 21. Working Drawing of a Worm and Worm-wheel 

course, always the same. A side view of the worm-wheel is 
sometimes included, particularly if the wheel is large and has 
arms or spokes. The notes on the drawing explain that the 
worm has a double right-hand thread and the lead and pitch 
are given. The notes for the worm-wheel give the number of 
teeth, circular pitch, the angle at which the wheel is held for 
gashing the teeth, etc. This angle for gashing would not be 
necessary in case the wheel were hubbed directly from the 



234 MECHANICAL DRAWING 

solid on a machine designed for this work and having a geared 
drive. 

Drawings of Spiral Gears. — - When two spiral gears are 
shown in mesh on a drawing, the teeth are simply represented 
by lines drawn diagonally but without attempting to incline 
them accurately. The information for cutting the gear is 
usually given partly in connection with the drawing itself, 
and partly in the form of notes. The drawing of a single 
spiral gear may be similar to the kind just mentioned, or it 
may be a sectional view and appear like the sectional view of 
a spur gear, the information needed for cutting the gear being 
given in notes. The drawing of a spiral gear should include, 
in addition to the outside diameter and other dimensions of 
the blank itself, the number of teeth; the tooth or helix angle 
(angle between tooth and axis of gear); whether the gear or 
helical tooth grooves are right hand or left hand; the diam- 
etral pitch of the cutter to use; the lead of the helix or 
spiral; and the whole depth of the cut. The combination of 
gearing to use for obtaining the required lead may also be 
included. 



CHAPTER X 
DESIGNING OR LAYING OUT CAMS 

Cams are used on different classes of automatic machinery 
and on various other mechanical devices, usually to secure 
mechanical movements which could not be obtained readily, 
if at all, by other forms of mechanism. Most cams rotate 
and the driven member has either a straight-line sliding move- 
ment or a swinging motion. Some of the most complex 
machines in existence are governed in their operation largely 
by means of cams which can be designed to give almost any 
action required. A cam has curved working surfaces which 
are laid out in accordance with whatever mechanical move- 
ment is necessary. The body of the cam may be in the form 
of a plate having a curved edge or it may be cylindrical and 
have a groove of the proper curvature cut into the cylindrical 
surface. These two general classes of cams are the most 
common although certain other forms are used. When lay- 
ing out a plate cam, the problem is to determine the shape of 
the edge and, in the case of a cylindrical cam, the curvature 
of the cam groove which engages a roller on the follower and 
gives it the necessary movement. The motion of the follower 
may be uniform or it may gradually increase and then gradu- 
ally diminish. The motion may also be irregular, as for 
example, when the follower has several periods of motion and 
rest during a revolution of the cam. In order to design or 
lay out cams, it is necessary to know how these different 
motions may be obtained by giving the working edge or groove 
of the cam the proper curvature. 

Designing a Cam for Uniform Motion. — The laying out 
of a cam for imparting a uniform motion to the follower will 
be illustrated by taking as an example a cam which causes 
the follower to rise a certain distance and then descend to the 

235 



236 



MECHANICAL DRAWING 



starting point. The diagram A, Fig. i, shows one half of the 
cam curve and illustrates the method of laying out this curve 
to secure a uniform motion. The center or axis of the cam 
is represented at a and the follower, at c. The follower in 
this case is shown as a pointed rod, although in actual prac- 
tice it is provided with a roller which bears upon the working 
edge of the cam, as will be explained presently. The vertical 
center line is first drawn and then a semicircle is described 




Fig. 1. Method of laying out a Heart Cam for a Uniform Motion 

about center a. In this case, the semicircle has a radius ab y 
point b representing the lower end of the stroke of the follower. 
Assuming that this stroke is to be 4 inches, this distance is 
laid off on the center line and is divided into any convenient 
number of equal parts, the number in this case being 8. The 
semicircle is also divided into the same number of parts. With 
a as a center, next describe an arc from division 1 on the 
center line intersecting radial line 1. Then describe an arc 
from division 2 intersecting radial line 2, and so on. These 
various points of intersection coincide with the curve of the 



LAYING OUT CAMS 237 

cam which is drawn through them. The other half of the 
curve (not shown in the illustration) is constructed in the 
same way. 

Cam Lay-out for Roller on Follower. — The general method 
of constructing a follower is to place a hardened steel roller 
at its end which bears on the edge of the cam as indicated by 
the diagram B, Fig. 1. The reason for using the roller is to 
reduce friction. Now the curve laid out as described in con- 
nection with diagram A represents the path followed by the 
center of the roller; therefore, when a roller is used, the work- 
ing edge of the cam must be inside of the curve shown at A 
an amount equal to the roller radius. Diagram B illustrates 
how the entire cam curve is laid out when a roller is used. 
The method followed is the same principle as described in 
connection with diagram A. In actually designing this cam, 
a circle representing the camshaft would be drawn first and 
then a larger circle for the hub of the cam. The center of 
the follower roller is next located at its lowest point which, 
in this case brings the roller into contact with the hub at one 
point so that the cam will not be larger than is necessary. 
In this example, the distance equivalent to the stroke of the 
cam happens to be laid off above the center of the roller, 
merely as a matter of convenience, instead of below the center 
of the camshaft as at A. The stroke is divided into a certain 
number of parts, the number in this case being 8. A series of 
concentric circles is then drawn through these division points 
and one half of the outer circle is divided into eight parts or 
into the same number as the stroke of the follower. From 
the point where the circle from division 1 intersects radial 
line 1, describe an arc equal to the radius of the cam roller; 
similarly, from the point where the circle through division 2 
intersects radial line 2, describe another arc equal to the 
roller radius, and continue describing these arcs with the 
intersecting points as centers, as indicated by the illustration. 
The cam curve is then drawn tangent to this series of arcs. 
Cams of this general class are commonly known as "heart 
cams" because of their shape. The follower is pushed upward 



238 



MECHANICAL DRAWING 




LAYING OUT CAMS 239 

positively by the cam, but it either descends by its own weight 
or is held in contact with the cam by a spring. 

Cam Lay-out wh^n Follower is Off Center. — In some cases 
the follower is offset relative to a vertical center line inter- 
secting the axis of the camshaft, as illustrated by diagram A, 
Fig. 2, and then the method of laying out the cam curve is 
modified somewhat. A distance on the center line of the fol- 
lower equal to the stroke is divided into a certain number of 
equal parts as in the previous example and these parts are 
numbered as before. From the center of the camshaft a 
series of concentric circles is drawn through these division 
points, and the outer circle is divided into twice the number 
of parts as the number of follower stroke divisions. A smaller 
base circle is also drawn from the center of the camshaft, 
which is tangent to the center line of the follower. Now in- 
stead of drawing radial lines as in the preceding examples, a 
series of lines is drawn from division points 1, 2, 3, etc., on the 
outer circle, which are tangent to the base circle. Arcs having 
a radius equal to the radius of the follower roller are then 
drawn from the points where these tangential lines intersect 
the concentric circles of like number, the same as described in 
connection with diagram B, Fig. 1. 

Cam Lay-out when Follower is Pivoted. — The follower of 
a cam is frequently in the form of a lever which is pivoted 
at the end opposite the cam roller; consequently, the follower 
swings about the pivot as the cam revolves and the center of 
the cam roller moves along the arc of a circle. The method 
of laying out the cam for a pivoted follower is illustrated at 
B, Fig. 2. The follower is pivoted at x and its upper position 
is indicated by the center line y. The arc representing the 
path followed by the center of the cam roller would, if ex- 
tended, intersect the axis of the camshaft. In order to lay 
out this cam curve, concentric circles are drawn through 
division points laid off on the stroke line of the follower as 
before. A large circle is then described about the center of 
the cam, which intersects the center x of the follower pivot. 
This circle is divided into 16 equal parts, or into twice as many 



240 



MECHANICAL DRAWING 




LAYING OUT CAMS 241 

divisions as the follower stroke line. With a radius equal to 
the length from center x to the center of the follower roller, 
arcs 1, 2, 3, etc., are described from the division points on 
the outer circle. The points of intersection between these 
arcs and the concentric circles of like number, are then used 
as centers for describing the small roller arcs which are tan- 
gent to the cam curve. 

Cam which has a Dwell or Rest Period. — The follower of a 
a cam does not always move continuously but may remain 
stationary during part of the cam's revolution. Diagram A, 
Fig. 3, illustrates how a cam is laid out to allow the follower 
to remain stationary during 60 degrees of the cam's revolu- 
tion. This rest period occurs at the top of the stroke, the 
follower being moved upward the full distance during 150 
degrees of the cam's rotation; it then remains stationary 
during a movement equal to 60 degrees, and then descends to 
the starting point. The stroke line of the follower is divided 
into a number of equal parts, as in the previous case, and 
concentric circles are drawn through these division points. 
Radial lines which are 60 degrees apart or 30 degrees from 
the vertical center line, are next drawn as indicated by the 
arrow marked "6o degrees." These lines represent the begin- 
ning and the end of the rest period and this part of the cam 
is concentric with the camshaft. That part of the outer 
circle not included between the sides of the 60-degree angle 
is divided into twice the number of equal parts as the number 
of division points on the follower line, there being 8 divisions 
for the 150-degree "rise" and a like number for the return 
curve. The cam curve is now laid out as described for pre- 
vious examples. s.9 

Cam Designed for Intermittent Movements. — The follower 
of a cam frequently has an irregular or intermittent movement 
with possibly several rest periods during onejprevolution of 
the cam. An example of a cam designed for giving the fol- 
lower an intermittent motion is shown by diagram B, Fig. 3. 
The total movement of the follower is assumed to be 6 inches 
as indicated by the six divisions on the vertical center line 



242 MECHANICAL DRAWING 

passing through the center of the roller. This cam is so 
laid out that the follower will rise 2 inches during 60 degrees 
of cam rotation, then rest during 30 degrees of rotation; there 
are two additional upward movements of 2 inches during 60 
degrees of cam rotation, each followed by rest periods of 30 
degrees; the follower then returns to the original or starting 
position during 60 degrees and remains there during 30 degrees 
of cam rotation. In designing cams of this kind it is conven- 
ient to divide the different movements of the follower accord- 
ing to the number of degrees of cam rotation as just described. 
As the periods of motion and rest are all 60 and 30 degrees in 
this case, the outer circle is divided into twelve parts of 30 
degrees each. The stroke of the follower is also divided into 
six equal parts in this case, and concentric circles are drawn 
through these division points. The cam curves are then 
laid out in the manner previously described. As the roller 
is to rise 2 inches in 60 degrees of cam rotation, the intersect- 
ing points between circles 1 and 2 and the 30- and 60-degree 
radial lines are used as centers for describing the roller arcs. 
As the follower is to rest during 30 degrees of cam rotation 
after rising 2 inches, that part of the cam curve between the 
60- and 90-degree lines is concentric. In the same manner 
the entire cam curve is completed. It will be noted that that 
part of the circle between the 270- and 330-degree divisions 
is divided into twelve parts and a similar number of radial 
lines are drawn. Arcs are also drawn between the concentric 
circles so that the number of radial lines corresponds with the 
number of circular lines. In this way, twice the number of 
centers are obtained for describing the roller arcs and conse- 
quently the contour or curve of this part of the cam can be laid 
out more accurately. Greater accuracy was considered necessary 
at this point because of the abruptness of the cam curve. In 
the practical designing of cams, the number of division points 
and circles must be regulated somewhat in accordance with 
the accuracy desired, which in turn depends upon the type of 
the cam and its contour. Thus the stroke, in this case, might 
have been divided into a greater number of divisions than six. 



LAYING OUT CAMS 



243 



Design of a Face Cam. — All of the cams previously re- 
ferred to have been of the type having curved edges which 
bear against the roller of the follower; hence, the follower is 
either held in place by its own weight or is assisted by means 
of a spring. This arrangement, however, is not always prac- 
ticable, because a positive drive in both directions may be 
desirable or necessary. One method of obtaining a positive 
drive is to make the cam in the form of a disk having a groove 



360 



270- 




160 



Machinery 



Fig. 4. Lay-out of a Face Cam which has a Groove engaging the Follower Roller 



of the required curvature cut into its face. The roller of the 
follower engages the groove as illustrated in Fig. 4 so that it 
is positively driven. This style of cam, which is sometimes 
called a " plate-groove cam," is laid out in the same manner 
as the plate or " periphery cams" previously referred to, 
except that a series of circles is drawn to represent different 
positions of the cam roller instead of the series of arcs, and 
then the curvature of the cam groove is drawn tangent to 
the inner and outer sides of these circles. This particular 



244 



MECHANICAL DRAWING 



cam has two rest periods each equal to 45 degrees of cam 
rotation. The periods of rise and fall are each equal to 135 
degrees and each of these periods is divided into the same 
number of equal parts as the number of divisions on the 
stroke line. 

Plate Cams Arranged for Positive Drive. — The face cam is 
not an ideal type because the outer and inner edges of the 
cam groove tend to rotate the roller in opposite directions 




Fig. 5. (A) Follower equipped with Two Rollers for obtaining a Positive Drive. 
(B) Illustrating Use of a Special Return Cam 



and the roller is also reversing constantly as it comes into 
contact with first one side of the groove and then the other; 
consequently, both the cam and roller are worn by the result- 
ing friction, particularly if the speed is rather high. To avoid 
these defects, some plate cams have followers equipped with 
two rollers in order to secure a positive drive. An example 
is illustrated at A, Fig. 5. Since the distance between the 
rollers remains constant, the cam must be designed so that 
the distance between any two points on the edge, as measured 
along some center line x-x or y-y, will be the same, which is 



LAYING OUT CAMS 



245 



necessary on account of the unvarying distance between the 
rollers. The term, "constant diameter cam," is sometimes 
applied to this type. The follower in this example is slotted 
to clear the camshaft which acts as a guide. Another kind of 
follower is in the form of a rectangular yoke which surrounds 
the cam; hence, the name "yoke cam" is applied to this type. 
These cams having rollers engaging both sides can be designed 
to give a definite motion only during 180 degrees of rotation, 
because the curvature of the remaining half must be such 





Machinery 



Fig. 6. A Cylindrical or Barrel Cam 

that the distance, as measured along any line intersecting the 
camshaft axis, will be the same at all points; furthermore, the 
curve of this remaining half, which represents the path of 
the follower roller, should not be nearer the camshaft axis 
than the curve for the first 180 degrees of cam rotation, 
because this will increase the motion of the follower. 

Use of a Return Cam. — By using a special return cam in 
addition to the main cam, as shown at B, Fig. 5, the limita- 
tions of the type of cam drive illustrated at A may be avoided. 
The follower may be placed between the main cam m and the 
return cam r and have rollers on opposite sides. When design- 
ing a cam drive of this kind, the main cam is first laid out to 



246 MECHANICAL DRAWING 

give the desired motion. From various points as a, b, c, d, 
etc., which represent the center of the cam roller in different 
positions relative to the main cam, radial lines are drawn and 
centers e, f, g, h, etc., are located diametrically opposite and 
at a distance equal to the center-to-center distance between 
the rollers. By describing roller arcs from these latter cen- 
ters, the contour of the return cam is determined. 

Design of a Cylindrical Cam. — Cylindrical or " barrel" 
cams, as they are sometimes called, are used when the mo- 
tion of the follower is parallel to the axis of the cam, although 
this type may also operate a pivoted follower. The diagram, 
Fig. 6, illustrates a cylindrical cam A which, as it revolves, 
imparts a reciprocating motion to the slide B that is assumed 
to be mounted in suitable ways. The stroke of the slide is 
equal to the distance measured parallel to the cam axis from 
the center of the curve at its extreme left-hand position, to 
the center at the extreme right. The cam curve may be laid 
out directly upon the cylindrical surface of the cam blank, 
but if a draftsman does the work, the usual procedure is to 
lay out a development of the curve and then transfer it to 
the cam. The development of the cam curve corresponds 
to the shape of the curve as it would appear if laid out upon 
a flat surface. 

The general method of laying out a cylindrical cam is illus- 
trated diagrammatically in Fig. 7. The rectangle shown at 
A is a development of the cylindrical cam surface, the height 
being equal to the length of the cam, and the length equaling 
the circumference of the cam. Upon this development a 
curve is drawn representing whatever shape will give the 
follower the required motion. This curve represents the path 
traced by the center of the follower roller. The principles 
governing the design of cam curves will be explained in detail 
later. At present we shall assume that the curve is com- 
pleted and that this curve is to be projected to the drawing 
of the cam which is shown at the left in Fig. 7. A circle rep- 
resenting a plan view of the cam is drawn and half the circle 
is divided into a certain number of equal parts, the number 



LAYING OUT CAMS 



247 



in this case being six. The base line of the development A is 
then divided into twice the number of parts as the half circle 
or, in this case, into twelve. Vertical lines are drawn from 
these division points from the base line of the development 
and other vertical lines are projected downward from the 
division points on the plan view. Horizontal lines are drawn 
from the points of intersection of the vertical lines at A and the 
cam curve, so that these horizontal lines intersect the vertical 



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Fig. 7. Lay-out of a Cylindrical Cam 

lines drawn from the division points on the circle. The points 
a, b, c, d, where the vertical and horizontal lines intersect, 
indicate the path of the cam curve. That part of the curve 
on the rear side of the cam which is indicated by the dotted 
line is laid out in the same way, except that the horizontal 
lines are drawn from the points where the right-hand half 
of the cam curve on development A intersects with the vertical 
lines. 

The development of a cam curve such as the one illustrated 
at A may be transferred to the cylindrical cam body by wrap- 

16 L 



248 MECHANICAL DRAWING 

ping the drawing itself about the cam and then marking 
various points along the curve upon the cam surface by means 
of a small prick-punch. From this curve, which is the center 
line of the cam groove, the groove itself is laid off to the re- 
quired width which depends upon the diameter of the cam 
roller. In order to reduce the friction and wear, the groove 
and roller of a cylindrical cam should be tapering. 

Cam Curves for Avoiding Shocks at High Speeds. — Some 
cams must be so designed that the follower is given a certain 
definite motion during the revolution of the cam. Heart cams 
are simple examples of this type as they are designed to give 
the follower a uniform motion throughout the cam's revolu- 
tion. The cam shown at B, Fig. 3, is another example. In 
this case the follower has certain periods of rest and of mo- 
tion during each revolution. When the follower does not 
require a certain kind of motion and the only condition is 
that the follower move a given distance during either a com- 
plete revolution or a part revolution of the cam, it is very 
essential to give the cam a curvature that is conducive to 
smoothness of operation, especially if the speed of rotation 
is rather high. The cam curves referred to in preceding ex- 
amples give the follower a uniform motion from the beginning 
to the end of its stroke; this uniform motion, however, is 
suitable only for comparatively slow speeds because, if the 
speed is increased beyond a certain point, shocks occur at the 
beginning and end of the stroke. To avoid these shocks 
and to secure smoothness of operation, other cam curves are 
employed. One of these is known as a simple harmonic motion 
and it corresponds to the motion derived from the well-known 
Scotch yoke or slotted cross-head, the follower being gradually 
accelerated to the maximum velocity and then gradually 
retarded. An action that is quite similar is obtained from 
an ordinary crank and connecting-rod, provided the latter is 
very long. Another form of cam curve which is even better 
than the harmonic motion curve is designed to give a uni- 
formly accelerated motion. The follower is accelerated from 
rest to maximum velocity and then retarded again at a uni- 



LAYING OUT CAMS 



249 



form rate which might be one inch per second, two inches 
per second, etc. In a slowly moving mechanism it might 
not be of much importance whether the cam operated the fol- 
lower with a uniform motion, a simple harmonic motion, or a 
uniformly accelerated motion; but when the speed of the 
cam is relatively high, the cam curvature must, if possible, 
be laid out to start and stop the follower as easily as possible 
to avoid shocks. Cams for high rotative speeds are designed 



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Machinery 



Fig. 8. (A) Cam Curve for Uniform Motion. (B) Crank or Harmonic Motion Curve 



to start the follower gradually, increase the speed to maxi- 
mum near the middle of the stroke and then gradually reduce 
it as the end of the stroke and the point of reversal are 
approached. 

Comparison of Uniform and Harmonic Motions. — The 
difference between a uniform and a harmonic motion is illus- 
trated very clearly by the diagrams in Fig. 8. The develop- 
ment of a line representing a uniform motion is illustrated at 
A. The rectangle represents part of the surface of a cylin- 
drical cam, the height being equal to the stroke and the length 



250 MECHANICAL DRAWING 

to the cam's circumference. If the cam were to be designed 
for an absolutely uniform motion, a distance x equal to one 
half the cam circumference would first be divided into a cer- 
tain number of equal parts. The vertical line equal to the 
length of the stroke would then be divided into the same 
number of parts. The intersecting points of vertical and 
horizontal lines drawn from these divisions would represent 
the path followed by the cam roller which, as will be seen, is 
a straight diagonal line for a uniform motion. Now, when 
the roller reaches the end of its stroke, it immediately starts 
downward at the same rate of speed and the result would be 
a shock unless the cam were operated very slowly. These 
shocks at the beginning and end of the stroke may be modified 
by curving the cam groove at the points of reversal, although 
this would shorten the stroke unless an allowance were made 
for the curved portions. 

The diagram B illustrates the simple harmonic motion 
curve which gives a much smoother action than the one de- 
signed for uniform motion. As will be seen, this curve passes 
through the intersecting points between horizontal and ver- 
tical lines. The vertical lines are drawn from equal division 
points laid off on a line equal in length to one half the cam's 
circumference; the horizontal lines, instead of being equally 
spaced, are drawn from division points on a semicircle, the 
diameter of which equals the stroke of the cam. 

Cam Curvature for a Uniformly Accelerated Motion. — If 
the speed of a cam is to be unusually high and a cam curve 
is desired that will give the smoothest operation, a curve 
known as a parabola is somewhat better than a curve giving 
a simple harmonic motion. In a uniformly accelerated, mo- 
tion, the distance which a body moves at the end of a given 
number of time units varies as the square of the number of 
such units. For example, if a body has a uniform accelera- 
tion of 2 inches per second, and D equals the distance, 
then Z>= J X2 X (i) 2 = 1 for the first second; D= J X 2 
X (2)2=4 for the next second, and so on. Uniformly retarded 
motion obeys the same law. Cam curves which are based on 



LAYING OUT CAMS 



251 



this law should preferably be used for cams designed for high 
rotative speeds, because the velocity of the follower increases 
uniformly from the beginning to the middle of the stroke, 
and then decreases uniformly; but with a crank curve the 
acceleration and retardation are slightly irregular. 











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Machinery 



Fig. 9. Methods of laying out Cam Curves for a Uniformly Accelerated Motion 

Laying out a Curve for Uniformly Accelerated Motion. — 

The method of laying out a curve for obtaining a uniformly 
accelerated motion is shown by diagram A, Fig. 9. This dia- 
gram shows only one half the curve, since the part for the 
return stroke is similar. The first step is to divide line x of 
the diagram or chart which, in this case, has a length equal 



252 MECHANICAL DRAWING 

to one half the cam circumference, into any convenient num- 
ber of parts, the number in this case being 6. One half of 
these division points are then numbered from right to left as 
at 1, 2, and 3. The square of this number (3) or 9 equals 
the number of parts that one half of the vertical line repre- 
senting the cam stroke should be divided into. Vertical lines 
are now drawn from division points 1,2, and 3 on line x and 
horizontal lines from divisions 1, 4, and 9 on the vertical or 
stroke line. The points of intersection between these vertical 
and horizontal lines coincide with the curve which is drawn 
through them. This part of the curve, which is one half of 
the curve for the forward stroke, gives a uniformly accelerated 
motion. It will be noted that vertical line 2 intersects with 
horizontal line 4, which is the square of 2, and that vertical 
line 3 intersects with horizontal line 9, which is the square of 3. 
The reason for this is, as previously explained, that a body 
having a uniformly accelerated motion passes over a distance 
during any number of time units equal to the square of the 
number of such units. That part of the curve for the uni- 
formly retarded motion is laid out as just described. It will 
be noted that the horizontal lines above the central division 
number 9 are spaced to correspond with those below it, or 
are the same distance from it. If a diagonal line is drawn 
between corners ab, it will represent the path of a follower 
which has a uniform motion. 

Modification of Curve for Pivoted Follower. — It is as- 
sumed that the cam curve shown at A, Fig. 9, is intended for 
a follower which has a straight-line motion parallel to the 
axis of the cam. If the cam roller is attached to a pivoted 
follower it will be necessary to modify the curve somewhat 
as indicated by the dotted line. The simple method of deter- 
mining how much to change the shape of the curve, is illus- 
trated at B. The vertical line x-x represents the axis of the 
cam and the arc represents the path of the roller on the fol- 
lower. From the points 1, 4, 9 (Diagram ^4), and correspond- 
ing points above point 9, lines are drawn between the vertical 
line x-x and the arc. The cam curve is then moved to the 



LAYING OUT CAMS 253 

right an amount determined by the lengths of these lines be- 
tween x-x and the arc. For instance, point d for the modi- 
fied curve is located to the right a distance equal to the length 
of line e. This modification of the curve compensates for the 
circular motion of the follower. 

Ellipse Method of Drawing Curve for Uniformly Accelerated 
Motion. — Diagram C, Fig. 9, shows how a cam curve may 
be drawn which is a close and satisfactory approximation for 
the uniformly accelerated motion curve. One of the hori- 
zontal lines representing one half the cam's circumference is 
divided into a certain number of equal parts and vertical 
lines are drawn from these division points. The horizontal 
lines which intersect with these vertical lines, thus indicating 
the path of the cam curve, coincide with divisions on an ellipse 
instead of using a semicircle as in the case of diagram B, Fig. 
8, which shows a curve for simple harmonic motion. It is 
evident that the kind of cam curve depends upon the propor- 
tions of the ellipse. The ratio of the major axis to the minor 
axis should be if to 1 or 11 to 8. 

Plate Cam Having Uniformly Accelerated Motion. — The 
method of laying out a plate cam so that the motion is uni- 
formly accelerated and retarded is illustrated at A, Fig. 10. 
This method is the same in principle as previously described 
in connection with diagram A, Fig. 9. Only one half of the 
cam curve is shown as the other half is similar and is laid out 
in the manner to be described. This cam is to move the fol- 
lower, during one half a revolution, a distance indicated by 
the arrow marked " stroke of follower." The stroke points 
are laid off on the vertical center line. A circle of any con- 
venient radius is next drawn and one half the circumference 
is divided into a number of equal parts; the number in this 
case is 6, which corresponds with the 6 divisions on line x of 
diagram A, Fig. 9. As 90 degrees represents the angle 
through which the motion is uniformly accelerated on the for- 
ward stroke, and as one fourth the cam circumference is 
divided into three parts, one half the cam stroke is divided 
into the square of 3, or into 9 parts and the remaining half, 



254 



MECHANICAL DRAWING 



into 9 parts. With the axis of the camshaft as a center, arcs 
are struck from points i, 4, and 9. The intersections of arc 
1 with radial line 1, of arc 4 with radial line 2 and of arc 9 with 
radial line 3, are points on that part of the cam curve which 
gives a uniformly accelerated motion on the forward stroke. 
The remaining part of the curve shown by diagram A is for 
uniformly retarding the motion and passes through the inter- 
secting points of arcs drawn from divisions above the central 









e 




IL 

O 
Ul 
O 

cc 
h 
a> 

) 


\ ^ FOLLOWER 


4- 


^^sL\. \ \ \\ 

v*\^ 2 ^ \ \ \\ 

c I \ 3 






W / »\ / 

\ ^c i\ / 

\y ^^ 1 / / 

— ^\ ^ Ss vj / / 11 
\ ir^l 1 

\ 1 '/// 
\ / / // 

\ //// 
\ / /// 


1 " 1 

7 


\ \ \ "A. I / / 

\ \ \y ^ / / 
\ N S\ "* \ 

\ J}* ^S. ^"^ I / / / 

— T \ \ W / // 

\\ z/ 

\ s yyy 
d\ yxy 

^\^yyy 






A 


| B 

Machinery 



Fig. 10. Plate Cams designed for Uniformly Accelerated Motion 

point 9 corresponding to divisions 4 and 1. It will be under- 
stood that this cam curve represents the center line of the path 
followed by the cam roller. When a follower is pushed up- 
ward by a cam of this type and then falls by its own weight, 
theoretically it should remain in contact with the cam because 
the principle of uniformly accelerated motion is the same as 
that of a falling body; in practice, however, the friction and 
inertia of connected parts would probably prevent the fol- 
lower from remaining in contact with the cam during the 
return movement unless the speed of rotation was quite slow. 



LAYING OUT CAMS 



255 



When the follower is pivoted and its motion is along an arc 
as shown at B, Fig. 10, the curve is laid out as described in 
connection with diagram A except that the radial lines which 
are intersected by the arcs are advanced somewhat. The 
equal divisions of the half circle corresponding to the radial 
lines shown at A are represented by dotted lines at B. The 
radial line a\ is advanced a distance 2#i equal to the distance 
4a between the vertical center line and the arc of the follower. 





x ? 


E^^SSaa-^ 


\ 


fff 




\ 


^ — \ 

t 


Z^^"^ / 5 


Machinery 



Fig. 11. How a Cam is laid out When it operates a Tangential Follower 



Similarly radial line b x is advanced a distance 3&1 equal to 9ft, 
and the advance of lines c h d h and e x is determined in the same 
manner. 

Plate Cam Having Tangential Follower. — The followers of 
some cams have straight or flat surfaces which are in a tan- 
gential position relative to the working edge or surface of the 
operating cam. The follower may be pivoted at one end (as 
indicated by the diagram, Fig. 11) or it may operate in guides 
and have a straight-line sliding motion. The pivoted follower 
will be considered first, the pivot being represented at A in 



256 MECHANICAL DRAWING 

Fig. 11. A distance equal to the stroke is first laid off on the 
vertical center line as in preceding examples, and one half the 
stroke is divided into 9 equal parts in this case, which is equal 
to the square of the number of divisions in one fourth of the 
circle. If the follower is to swing about A as a center, it will 
assume positions 1, 2, 3, etc., as the radial lines 1, 2, 3, etc., 
swing around to a vertical position. The points of intersection 
1, 2, 3, and so on, on the radial lines are found just as de- 
scribed in connection with diagram A, Fig. 10, but instead of 
drawing the cam curve through these points, straight lines 
representing the edge of the follower must be drawn from 
them. The angle between any straight line and its correspon- 
ing radial line must be equal to the angle which the radial 
line makes with the follower when it swings around to the 
vertical position. For instance, when radial line 4 is in a 
vertical position, the follower will be at x; therefore, angle b 
should be equal to angle a. After this series of lines is drawn 
from points 1, 2, 3, 4, etc., the cam curve is drawn tangent to 
them. Now, if the follower instead of being pivoted moves 
up and down with its working face always parallel to the first 
position, then all of the angles b between the radial and straight 
lines will be right angles. The working surface of a cam hav- 
ing a tangential follower must be entirely convex because any 
concave sections of the curve would not come into contact 
with the follower. 



CHAPTER XI 

GEOMETRICAL DRAWING PROBLEMS AND THE DEVEL- 
OPMENT OF INTERSECTING SURFACES 

Mechanical draftsmen should know how to construct the 
different geometrical figures and curves which are used in 
drafting practice. The draftsman's methods of constructing 
certain of these geometrical figures differ from the procedure 
explained in many text-books, both on geometry and mechani- 
cal drawing, because the draftsman uses instruments that 
enable the work to be done more rapidly. While such figures 
as the square, hexagon, octagon, and many others may be con- 
structed by means of*a straightedge and compass, the drafts- 
man's method is simplified by using the T-square and triangles 
or possibly a universal drafting machine. For example, the 
sides of a hexagon are drawn by using the 30-60-degree tri- 
angle and without first dividing a circle by means of a com- 
pass; or if one line must be perpendicular to another, it is 
drawn by using a T-square and triangle, rather than by the 
slower methods which are explained in books dealing with 
plane geometry. While these geometrical principles should 
be thoroughly understood, only a few of the more important 
problems will be considered in this chapter, as it is assumed 
that most of the users of this book are familiar with simple 
problems, such as bisecting a line, erecting a perpendicular 
to a line at a given point, etc. Moreover, there are many 
text-books which may be referred to for this elementary 
information. 

Drawing an Ellipse. — When a circle is represented on a 
drawing in an oblique position, which is quite common, it 
has the form of an ellipse. There are several ways of draw- 
ing an ellipse, some being accurate and others approximate 
but rapid. The draftsman ordinarily has the maximum and 

257 



2 5 8 



MECHANICAL DRAWING 



minimum dimensions of the ellipse, which are known, respec- 
tively, as the major axis and minor axis. 

An accurate method of drawing an ellipse is illustrated in 
Fig. i. Two circles are first described from a given center. 
The diameter of the larger circle is equal to the major axis 
of the ellipse, and the diameter of the smaller circle is equal 
to the minor axis. From various points, as at A, B, and C, 
on the outer circle, radial lines are drawn which intersect the 
inner circle at a, b, and c. Horizontal lines are drawn from 
the points a, b, and c, and vertical lines from the points A y B y 





— - ^A 

7 oXA\ /\ / 
/ / \ ^>vK c / \ / 


c 

° A 1 B 




\ xT/S^y V /x* \ 
\/><a/ X/ \f \ 


D 

E 

Machinery 



Fig. 1. Accurate Method of drawing 
an Ellipse 



Fig. 2. Approximate Method of drawing 
an Ellipse 



and C. The intersections of these horizontal and vertical 
lines are points on the curve of the ellipse which is drawn 
through them. 

Approximate Method of Drawing an Ellipse. — Many of 
the methods of drawing an ellipse approximately are com- 
plicated and difficult to practice, and some of them do not 
give good results unless the ratios of the major and minor 
axes are within certain limits. Figure 2 illustrates a simple 
way of drawing an ellipse, which is sufficiently accurate for 
many purposes. The distance AB represents the major axis 
and the distance CD, the minor axis. First draw the diagonal 
line AD and then locate point E a distance from O equal to 
one half the major axis; then, with D as a center, describe 



GEOMETRICAL DRAWING PROBLEMS 



259 



the arc EF. Point H is next located midway between A and 
F and a perpendicular line EL is drawn, thus locating points 
K and L. From these two centers, the large and small arcs 
for forming one half of the approximate ellipse are described. 
The centers for forming the other half of the ellipse are located 
in a similar manner. 

Straightedge or Trammel Method of Drawing an Ellipse. — 
The method of drawing an ellipse, illustrated at the upper 




Fig. 3. Straightedge or Trammel Method of drawing an Ellipse — The Ellipsograph 

part of Fig. 3, is accurate and illustrates the principle govern- 
ing the operation of the ellipsograph shown in the same illus- 
tration. On a straightedge, which may be made of a strip of 
stiff drawing paper, three lines, a, b, and c, are located. The 
distance ac equals one half the major axis of the ellipse to be 
drawn, and the distance be, one half the minor axis. Hori- 
zontal and vertical center lines are first drawn, and then 
various points on the ellipse are located by moving the straight- 
edge along from one position to another, keeping line b on the 



26o 



MECHANICAL DRAWING 



major axis and line a on the minor axis. For instance, when 
the straightedge is in the position shown by the dotted lines, 
point c coincides with the path of the ellipse and it also coin- 
cides with the ellipse when the straightedge is in any other 
position, provided points a and b are in line with their respec- 
tive horizontal and vertical center lines. When drawing an 
ellipse by this method, various points are first located and are 
marked by a series of dots opposite graduation c for different 
positions of the straightedge. A draftsman's curve is then 
used to draw the ellipse through these various points. 





1 2 3 4 5 6 7 I 


5 


T*z. ^\ 


r 

i 

i 


//A— A 








I 

i 


\X7^ \ 


. yy \ 




Machinery 



Fig. 4. Method of constructing a Helix 



The ellipsograph shown in the lower part of Fig. 3 has a 
graduated bar d (graduations not shown) and is adjustable 
for drawing ellipses having major axes varying from | up to 
22 inches. Such an instrument is desirable if this work must 
be done frequently. Several other devices operating on this 
same principle are in use. Sometimes a beam trammel having 
three points instead of two is used in conjunction with a 
square. In this case, two of the points corresponding to 
graduations ab (Fig. 3) are held in contact with a square, 
while the third point describes the ellipse. Of course only 



GEOMETRICAL DRAWING PROBLEMS 



261 



one fourth of the ellipse is drawn at a time, and then it is 
necessary to shift the position of the square. 

Construction of a Helix. — The helix, which is frequently 
but incorrectly referred to as a " spiral," is represented by the 
curve of a screw thread. The method of drawing a helix is 
illustrated in Fig. 4. One half of the circumference of the 
cylinder, on the surface of which the helix is to be described, 




Fig. 5. Representing a Large Square Thread by drawing Accurate Helical Curves 

is divided into a number of equal parts. In this case there 
are eight divisions as shown at the left-hand side of the illus- 
tration. One half the lead of the helix (the lead equals the 
distance that the helix advances in one revolution) is divided 
into a similar number of equal parts. Horizontal lines are 
next drawn from the division points 1, 2, 3, etc., on the circle 
and vertical lines from the other division points. The inter- 
sections between the horizontal and vertical lines which are 



262 



MECHANICAL DRAWING 



numbered alike, represent points on the helical curve, as the 
illustration shows. 

The application of this method to the drawing of a large 
square thread is shown in Fig. 5. In this case there are two 
helical curves, one representing the outer edge of the thread 
and the other the inner corner or root; therefore, two half 
circles are drawn. The radius of the larger circle is equal to 
the outside radius of the screw, and the radius of the smaller 
circle is equal to the radius of the screw at the root of the 
thread. The intersecting points for the outer edge of the 
thread are obtained by projecting lines from the outer circle, 




Fig. 6. Diagrams illustrating the Involute Curve 



whereas the points along the inner helix are obtained by pro- 
jecting lines from the smaller circle. If a number of threads 
were to be drawn, as might be required on a drawing used 
for illustrating purposes, paper templets could first be laid off 
by the method illustrated in Fig. 5 and these templets used 
for drawing the helical curves on the different threads. 

The Involute Curve. — The involute curve (or, simply, the 
involute) may be defined as a curve which may be traced by 
a point, as a pencil, fixed to the end of a flexible cord, when 
unwound from the surface of a cylinder. This curve is rep- 
resented by the line A in Fig. 6; B is the generating cylinder 
or circle from which the cord is supposed to have been un- 
wound. For ordinary purposes this curve may be approxi- 



GEOMETRICAL DRAWING PROBLEMS 



263 



mately drawn with a generating circle of a given diameter 
by the following method (see the right-hand figure): Within 
the generating circle B inscribe a square, extending the lines 
forming its sides, as shown, to C, D, E and F. With the 
point G as a center, and the distance GK as a radius, scribe an 
arc extending from the line FG to the line CG. With H as a 
center, extend the dividers to the point where the last arc 
ended on the line CG, and continue the curve to the line DH. 
With J as a center, and the radius extended to the intersection 
of the curve on the line DH, continue the curve to the line 
EJ. Continue these opera- 
tions until a spiral of suf- 
ficient length has been 
produced. 

If this curve is generated 
on a large scale a greater 
number of basic lines, as 
CG, DH, etc., should be 
made, so as to increase the 
accuracy of the curve. It 
should be understood that 
an accurate involute curve 
cannot be described with the dividers, since no part of it is 
the arc of a circle. In drawing this curve mechanically, for 
instance, as a curve on large wood pattern work, a generating 
cylinder of wood of proper diameter is used, and around this 
is placed a very fine copper wire, to the end of which a draw- 
ing pencil is fastened. By this means very accurate work 
can be done. It may be well to state that, mathematically, 
the diameter of the generating circle multiplied by 3. 14 16 will 
give the distance between one convolution and the next, as 
shown by the distance M. 

Drawing an Involute Curve Accurately. — The involute 
curve is the basis of the involute system of gearing, although 
on ordinary gear drawings it is not necessary to draw the tooth 
curves accurately, and they need not be drawn at all. In 
many cases the gear teeth are represented by dotted circles 




Fig. 7. 



How to draw an Involute Curve 
accurately 



264 MECHANICAL DRAWING 

the diameters of which equal the pitch diameter, outside diam- 
eter, and root diameter. If a few teeth are drawn where the 
two gears mesh, the draftsman does not attempt to draw 
the curves accurately, as this is not necessary since the shape 
of the teeth is governed by the gear-cutting process. 

Figure 7 illustrates how a small part of an involute curve 
may be accurately drawn in case this should be necessary. 
The generating circle in this case is comparatively large. The 
intersection of the vertical center line AB with the generating 
circle marks the beginning of the involute curve from this 
point; toward the right a number of equal spaces are laid 
off and these are numbered from left to right. Radial lines 
are drawn from each of these points to the center B of the 
generating circle. By using a straightedge and a 90-degree 
angle, tangent lines at right angles to each of these radial lines 
are drawn. These tangents are given numbers corresponding 
to the numbers of the radial lines. With point 1 on the gen- 
erating circle as a center, and the distance from this point to 
the vertical center line as a radius, that part of the involute 
curve between the generating circle and tangent line 1 is 
described. Then with point 2 on the generating circle as a 
center, and with the distance from this point to the termina- 
tion of that part of the curve just drawn, as the radius, that 
section of the involute curve extending between tangent lines 
1 and 2 is described. By proceeding in the same manner and 
using points 3, 4, 5, etc., on the generating circle as centers, 
the remainder of the involute curve is generated. When lay- 
ing out an accurate curve, it is essential, of course, to use a 
very sharp-pointed pencil and to draw the curve on a smooth 
surface. In the application of this method, there may be 
from twelve to twenty tangent lines in one fourth of the cir- 
cumference of the generating circle, although only from one 
fifth to one eighth of the circle may be required for generating 
an involute curve of the desired length. 

Drawing Cycloidal Curves. — If a circular disk is rolled 
along a straightedge, a point on the disk will describe a curve 
known as a cycloid. If this disk or " generating circle" is 



GEOMETRICAL DRAWING PROBLEMS 



265 



rolled upon the outer surface of another disk, a given point 
will describe an epicycloid; if the disk is rolled around the 
inner surface of a ring, the point will describe a hypocycloid. 
These epicycloidal and hypocycloidal curves are the basis of 
the cycloidal system of gearing, which has been largely re- 
placed by the involute system. 

The Epicycloid. — To describe an epicycloid upon a given 
circle AB (Fig. 8) with a generating circle C proceed as fol- 
lows: Through the centers of the given circles draw the 





j 


R____ 




\J^-— 




2 jSS 


r~~7 




1 2 



Machinery 



Fig. 8. Generating an Epicycloidal Fig. 9. Generating a Hypocycloidal 

Curve Curve 

vertical line EF, the intersection of which with the circles 
determines the beginning of the required curve. From this 
point lay off toward the left on the generating circle C, and 
toward the right on the given circle AB,sl number of equal dis- 
tances, numbering the points i, 2, 3, etc., as shown. Through 
the center of the generating circle, and with the center F of the 
given circle as a center, describe the arc GH, upon which the 
center of the generating circle will travel as it rolls along. 
Through the points 1, 2, 3, etc., on the given circle draw the 
radiai lines /, K, L, etc., intersecting the arc GH. Through 



266 MECHANICAL DRAWING 

the points i, 2, 3, etc., on the generating circle, and with F 
as a center, describe the arcs 0, P, Q, R, etc. With the radius 
of the generating circle C, and the intersections of the radial 
lines J, K, L, etc., with the arc GH used successively as 
centers, describe short arcs intersecting the arcs 0, P, Q, etc. 
These intersections are points on the required epicycloidal 
curve. 

The Hypocycloid. — ■ A hypocycloid is generated in the same 
manner as an epicycloid, except that the generating circle 
rolls on the inside of another circle. It is constructed as fol- 
lows: Draw the vertical line EF (Fig. 9) through the centers 
of the given circle AB and the generating circle C, the point 
of intersection of this line with these circles determining the 
beginning of the required curve. With the center F of the 
given circle AB as a, center, describe the arc GH passing 
through the center of the generating circle C. Divide the 
generating and given circles into equal spaces, and through 
the points on the given circle draw radial lines intersecting 
the arc GH. With the center F of the given circle as a center, 
describe the arcs O, P, Q, P, etc., passing through the points 
1, 2, 3, etc., on the generating circle. With the radius of the 
generating circle C, and the intersections of the radial lines 
with the arc GH used successively as centers, describe short 
arcs intersecting the arcs O, P, Q, R, etc. These intersections 
are points on the required hypocycloidal curve. 

Development of Intersecting Surfaces. — The use of the 
term " development " in connection with drafting practice 
may be illustrated by considering a practical example. If a 
steel sheet forming one section of a boiler is unrolled and 
flattened, it will represent a development of that particular 
section. If the rolled sheet formed a cylindrical part of the 
boiler, the development of this sheet would be a rectangle, or 
possibly a square. When one cylindrical body intersects or 
is fitted to another, the edges which fit together have a cer- 
tain curvature, as seen on a development of the intersecting 
surface. When the cylindrical bodies are formed of sheet 
metals, it is necessary to determine the shape of this curve 



GEOMETRICAL DRAWING PROBLEMS 



267 



when the sheet is flat or before it is rolled to shape. A pattern 
may be laid out first which represents a development of the 
sheet, and then the steel plate may be cut to the shape of 
this pattern or templet so that, when it is rolled to cylindrical 
form, it will accurately fit the other cylindrical body. Such 
patterns are used in boiler shops and in other places where 
parts are made from sheet metals. 

The usual problem is to lay out a development of the inter- 
secting surfaces from a drawing of the finished object. Cer- 
tain allowances have to be made in actual practice for forming 




Fig. 10. Development of the Pattern for an Elbow 

lap joints for riveting parts together, and it may be necessary 
also to allow for changes in length, due to bending of the sheets 
when the latter are comparatively thick or heavy. These al- 
lowances will not be considered in explaining the methods of 
developing patterns, the assumption being that the edges 
butt together at the joints. 

Development of an Elbow. — An elbow, such as is formed 
of two cylindrical pipes which intersect at right angles to 
each other, is a typical example of sheet-metal pattern de- 
velopment. An elbow of this kind is shown at the left in Fig. 
10. It is assumed that both sections of the elbow are of the 
same diameter. The diagonal line AB represents the joint 



268 



MECHANICAL DRAWING 



as seen in the side view. If one of these sheets is unrolled or 
flattened, it will appear as shown to the right in the illustra- 
tion. The problem is to determine the shape of the curve 
G, K, H. The first step is to divide the end view or circle 
corresponding to the size of the elbow into a number of equal 
parts, which should preferably be divisible by 4. The num- 
ber of divisions depends upon the degree of accuracy required 
in developing the curve. From these division points, vertical 
lines are drawn which intersect with the diagonal line AB. 




Fig. 11. Development of a Three-piece Elbow 



The base line EF of the pattern or development is made equal 
to the circumference of the elbow, neglecting any allowance 
for a lap joint, and the height EG and FH is equal to length 
AC. Line EF is next divided into as many equal spaces as 
the circle, or into twelve equal spaces in this case. Vertical 
lines are then drawn through each of these division points on 
line EF. The point where line No. 2 from the circle inter- 
sects line AB is now projected over to line No. 2 on the de- 
velopment; thus locating one point a oi the curve. In this 



GEOMETRICAL DRAWING PROBLEMS 



269 



same manner, the other points b, c, and d, etc., are located. 
In other words, the vertical distances 2a, 36, etc., are equal 
to the distances between the base line CD and the diagonal 
line AB on lines 2, 3, 4, etc. After the points in one half of 
the curve GK are located, the corresponding points on the 
remaining half are determined in the same manner and the 
curve is then drawn through these locating points. 

Development of a Three-piece Elbow. — A three-piece 
elbow is illustrated in Fig. 11, and the problem in this case is 
to develop a pattern for the central section. A circle repre- 
senting an end view of one section of the elbow is first divided 




Machinery 



Fig. 12. Development of the Pattern for the Dome Sheet of a Boiler 



into a number of equal divisions, as in the preceding example, 
and radial lines are drawn through these points. Vertical 
lines are also drawn which intersect with line EF, and through 
these points of intersection other lines are drawn parallel to 
the sides DE and CF. After a center line NO is drawn, a 
distance is laid off equal to the circumference of the elbow, 
and this distance is divided into as many equal spaces as the 
circle which, in this case, is twelve. A center line LM is also 
drawn which bisects the angle formed by the sides CD and 
EF. The distance between this center line and the line EF 
as measured on lines 1, 2, 3, etc., is transferred to the develop- 
ment, care being taken to lay off the distance in each case on 
lines which are numbered alike. In this way points for the 



270 



MECHANICAL DRAWING 



lower edge are determined, and as the upper edge is similar, 
it is simply necessary to lay off the same distances above and 
below the center line NO. The principle of this " parallel- 
line" method of development is applied in many of the prac- 
tical problems encountered in sheet-metal pattern-drafting. 

Intersecting Cylinders of Unequal Diameter. — A problem 
in pattern development, which is the same in princiole as the 




Machinery 



Fig. 13. The Radial-line Method applied to a Pattern Development 



ones previously referred to, is illustrated by the diagram, 
Fig. 12, which shows how the pattern for the dome sheet of a 
boiler is laid out. t In this case, the cylindrical dome is much 
smaller in diameter than the boiler. A circle representing a 
plan view of the dome is divided into a number of equal spaces 
as before, and vertical lines are projected downward from the 
intersecting points. A distance equal to the circumference 
of the dome is laid off on line AB. After dividing this line 



GEOMETRICAL DRAWING PROBLEMS 27 1 

into the same number of equal spaces as the dome circle and 
drawing vertical lines from these division points, line a is made 
equal to line a h line b equal to line h, etc. After laying off 
line d, the next line c on the pattern is made equal to Ci, the 
next equal to b h and so on. The other half of the pattern is 
then laid out in the same manner. The dotted line around 
the pattern indicates the allowance that would be made for a 
lap or flange by means of which the dome would be riveted 
to the boiler shell. 

Development of Conical Shapes. — All of the developments 
referred to previously have depended upon measurements of 
distances on parallel lines projected from equal division points 
on a circle. When the development is of a conical body, 
radial lines are employed instead of parallel lines. If the 
development of a cone were required, it would simply be 
necessary to describe an arc having a radius equal to the 
length of the side ab of the cone (see diagram A, Fig. 13) and 
a length be equal to the circumference of the base of the cone. 
Straight lines from points b and c at the extremities of the arc 
to the apex a complete the development. The flat surface 
dgcb represents the development of a frustum of a cone hav- 
ing sides fedb. 

The diagram B, Fig. 13, shows the development of a cone 
intersected by a plane ge which is at an angle with the base 
be. One half the circle representing the end view of the base 
is first divided into a number of equal parts, the number in 
this case being six. Vertical lines are drawn from these points 
which intersect the base line be, and from these intersecting 
points the lines are extended to the apex a. The arc cd which 
forms part of a complete development of the cone is divided 
into twice the number of spaces as the semicircle representing 
the cone base. Radial lines are drawn from these division 
points on arc cd to the apex a. On these radial lines points 
are located which correspond to the curvature from e to /. 
The intersecting points between line ge and the lines extend- 
ing from the apex to the cone base are projected to the side 
ac by drawing horizontal lines as shown. At the termination 



272 MECHANICAL DRAWING 

of each horizontal line, an arc is struck from the apex a as a 
center. That arc which represents the continuation of line 
No. i from the base should intersect with line No. i on the 
development; the arc which is a continuation of line 2 on the 
base should intersect radial line 2 on the development, and so 
on for the other numbers. The curve is then drawn through 
these various points of intersection. If the pattern developed 
in this way were rolled to cylindrical form, it would have a 
circular base and an inclined top edge, as indicated by the 
section of the cone bgec. 



CHAPTER XII 

DRAFTING-ROOM SYSTEMS, EQUIPMENT, AND 
ARRANGEMENT 

In every drafting-room it is necessary to have some sys- 
tem that will make it possible to locate readily and to identify 
all drawings and records, in order to supply the shop quickly 
with whatever information may be needed, and also to enable 
duplicate machines to be constructed at any time. The sys- 
tem generally includes a method of filing tracings, card indexes 
for locating them readily, a record of blueprints which have 
been sent out, and other features, the system varying some- 
what according to the size of the drafting-room and the variety 
of existing drawings and records. 

The drafting-room may also include in its organization men 
who are competent to direct manufacturing operations and to 
specify on " operation sheets" just how each part of a mecha- 
nism is to be machined, so that the shop man can confine his 
attention to the actual work of construction. The reason for 
the adoption of this practice is to avoid having the same 
kind of work done in various ways and according to the ideas 
of different men, the object being to specify what seems to be 
the best method, after the conditions in each case have been 
carefully studied. Owing to the special forms of tools and 
fixtures now used, the designer (often with assistance from the 
shop) must of necessity control or determine beforehand how 
such tools are to operate, since the design of such special equip- 
ment is based upon the method of operation. The draft- 
ing department, however, may or may not decide as to the 
order of operations and machines to be used when standard 
tools are employed. The relation between the drafting and 
manufacturing departments varies widely and depends upon 
the size of the plant, the nature of the product manufactured, 

273 



274 MECHANICAL DRAWING 

and the ideas of the management in regard to methods of 
planning and controlling manufacturing operations. 

Just how extensive the system of a drafting-room needs to 
be depends largely upon the variety of the work manufactured 
and to what extent the activities of the draftsmen extend 
beyond the work of machine design and mechanical drawing. 
This chapter will deal with some of the different methods of 
recording, locating, and identifying drawings or patterns, and 
of increasing the effectiveness of the drafting-room either by 
means of its system, arrangement, equipment, method of 
lighting, or any other features which tend to promote the 
work of the draftsmen. 

General System of Filing Drawings. — As it is the cus- 
tomary practice to use at least two or three different sizes of 
drawings, tracings of the same size are usually filed together, 
because it is inconvenient to find them when large and small 
sheets are placed together in a drawer, unless the drawer is 
large enough to take different sizes which are separated by 
partitions. According to a common method, the different 
standard sizes of sheets which have been adopted are repre- 
sented by letters placed beside the drawing number. For in- 
stance, if the standard sizes are 24 by 36 inches, 18 by 24 
inches, 12 by 18 inches, and 9 by 12 inches, the letter A might 
represent the 24- by 36-inch size, the letter B, the 18- by 
24-inch size, etc. The drawing number which accompanies 
the letter may also indicate the location of the drawing in the 
filing cabinet. To illustrate, suppose the number of a drawing 
is B315. In this case, the letter B shows that the drawing is 
an 18- by 24-inch size, the first figure of the number shows 
that it is in drawer No. 3, and the remaining figures, that it 
is the fifteenth drawing in this particular drawer. Possibly 
the drawing would be placed in a folder marked with the name 
of the machine for which the drawing was made. If the 
drawing happens to be a detailed view of a part used on more 
than one machine, blueprints are made and marked " record 
prints. " These prints are then filed the same as the tracings 
wherever this particular detail is required. In this way, a 



DRAFTING-ROOM MANAGEMENT 



275 



complete set of drawings for each machine is always on hand 
and convenient for reference. Instead of designating the size 
of the drawing by a letter accompanying the drawing num- 
ber, certain blocks of numbers may be assigned to each size. 
The range of numbers allowed for each size is large enough to 
last for years. With this system, the size of the drawing 




Fig. 1. A Numerical Card Index for Drawings 



number indicates the size of the sheet, to one who knows the 
sizes corresponding to the different blocks of numbers. 

Card Index for Locating Drawings. — When a great many 
drawings are tiled in a cabinet, it would usually be difficult 
to locate different tracings without some system of indexing, 
and for this reason what are known as "card indexes" are 
commonly used. These indexes serve the same purpose as 



276 MECHANICAL DRAWING 

the index of a book. The system of indexing suitable for one 
drafting-room may not exactly meet the needs in another 
where different conditions exist in the plant. For this reason, 
the particular systems referred to may not be directly appli- 
cable in all cases, but the general principle can be applied 
with whatever modifications may be necessary. 

A common form of index consists of cards which represent 
the drawings and which are filed according to some order. 
One form of card index is illustrated in Fig. 1. This is some- 
times called a " numerical index" because the cards are filed 
with reference to the letters and drawing numbers in consecu- 
tive order. All the cards for drawings of the "A" size are 
placed back of the marker card A, the cards for the "B" size 
are placed back of the marker card B, and so on. If the num- 
ber of a drawing is B315, the card for this drawing would be 
filed in section B and after card No. 3 (see Fig. 1). These 
cards should be numbered consecutively and in advance to 
avoid the possibility of having the same number on two draw- 
ings, as this index must be referred to before numbering a 
drawing. 

For example, if a size A drawing for drawer No. 2 is to be 
numbered, and by referring to the index under A2 it is found 
that A2 10 is the last number used, this shows that the draw- 
ing should be No. A211. This number is placed on the draw- 
ing, and on the index card is printed the name of the part 
represented by the drawing, the date, the pattern or piece 
number, and possibly other information, such as the name 
or symbol of the draftsman, the checker, as well as special 
remarks when necessary. This form of card index enables 
one to locate readily any drawing, provided the number is 
known. When it is necessary to locate the drawing by re- 
ferring to the name of the part drawn, a different card index 
is used. 

Card Index Based on Classes of Machines. — The greatest 
difficulty in devising a satisfactory index system for drawings 
is encountered in shops which handle a great variety of work. 
On the contrary, the problem is relatively simple in the draft- 



DRAFTING-ROOM MANAGEMENT 



277 



ing-rooms of shops where the product is limited to only one 
or, perhaps, a few standard machines or tools which are manu- 
factured in large quantities. In order to provide a more 
complete system of indexing than would be available where 
only a numerical index is used, many drafting-rooms have a 
card index arranged alphabetically with reference to the 
different classes of machines or tools represented by the draw- 
ings. This system may be used in conjunction with the 




Machinery 



Fig. 2. Card Index based on Classes of Tools or Machines 

numerical index previously described, or it may be considered 
satisfactory without an additional index arranged according 
to drawing numbers. 

Figure 2 illustrates how the card index is arranged when 
the cards are filed with reference to different classes of tools 
or machines. This particular illustration shows that part of 
the file which is used for cutters. In this case, the cards for 
drawings of all classes of cutters are placed back of the head- 



278 MECHANICAL DRAWING 

ing "Cutters"; sub-headings should be added to the index if 
the variety of cutters is great enough to warrant subdivision. 
In the same way, the cards for machines should be filed ac- 
cording to the general classes of machines represented, and 
all the attachments for such machines should be indexed under 
the heading of the particular machine for which the attach- 
ment is intended. For instance, the card for the dividing 
head of a milling machine should be indexed under " milling 
machine" and back of the subdivision marked " dividing head." 
Special tools, such as jigs and fixtures, that are used for 
manufacturing operations on parts of standard machines and 



Drawing No. A-613 Date March 


6 , 1905 


Drawn by M. C-r Checked by 


Potter 


Casting Detail: 

Special head for #2 




Blank & Blank Milling Machine. 




Piece No. 656 




Remarks: For construction see A-109. 




For milling hexagon nuts. 





Fig. 3. A Typical Index Card 

tools are indexed in the same divisions as the parts on which 
the operations are to be performed. For example, the fixture 
for drilling the dividing head of a milling machine would be 
indexed under " dividing head" back of the main division 
headed "milling machine." Whenever it is difficult to decide 
under which heading a certain tool or fixture should be in- 
dexed, it is advisable to make out two or more cards and place 
them in those parts of the file where they are most likely to 
be looked for. 

Figure 3 shows a typical index card. The piece number 
given on the card is also the pattern number if the part is a 
casting. When the pattern numbers are marked not only 
on the drawing, but on the index card, a special index of pat- 



DRAFTING-ROOM MANAGEMENT 279 

terns is not needed, although it is convenient and in many 
cases necessary to be able to tell from the number of the 
pattern for what machine or tool this pattern was used. For 
this reason, a book or pattern register should be provided in 
which the pattern numbers are arranged in succession, the 
number being entered in the book as soon as the drawing is 
made. 

Card Index for Jobbing Shop. — In a jobbing shop handling 
a general line of repair work, and possibly building new ma- 
chinery, there should be some system of keeping records of 
machines and parts of machines sent out, as well as of the 
drawings. In most shops of this kind, some of the work is 
made according to blueprints or sketches supplied by the 
customer, and other work is made from drawings obtained 
from the drafting-room of the jobbing shop. Similarly, the 
patterns may be obtained from a customer or be made by the 
firm. This system should be so arranged that the drawing for 
any part of any machine may be readily found if the customer's 
name is known. There should also be a complete index of all 
patterns, drawings, outside blueprints, etc., to avoid a dupli- 
cation of work in connection with future orders. 

According to the system to be described, when an order is 
received from the customer it is written out on a form, dupli- 
cate copies of which are sent to the pattern shop, boiler shop, 
machine shop, or wherever work is to be done. We will sup- 
pose that this order is for a machine to be made to the firm's 
own drawings. The drawing office then, on receipt of this 
order, makes out a production sheet on bond paper forms, 
giving name and number required of each part, drawing num- 
ber, pattern number if a casting, and material of which it is 
made. This production sheet should include everything re- 
quired such as bolts, nuts, oil-cups, gaskets, split pins, name- 
plate and every detail, no matter how small. In the case of 
forgings it should give, in addition to drawing number, the 
length and size of bar required to make it. The required 
number of prints should then be made from the production 
sheet. The order number, name of customer, date issued and 

18 L 



280 MECHANICAL DRAWING 

number of machines required (the production sheet should 
always be made out for one machine only) should be marked 
on the prints, and not on the original, as this may be used 
again later, on other orders. One print should then be sent 
to the stores department, to order the material from, and one 
to each department having work to do on that order. Also 
one print should be filed away as a record under the order 
number, preferably in an envelope, together with any special 
specification or other matter referring to that order only; these 
should be kept in numerical order and be stored in a fireproof 
vault, but in a convenient place for reference. The original 
can now be altered to suit any future orders or improvements 
in design without affecting the record of that order. Any 
alterations to the drawings for subsequent orders are also 
made in such a way that there is a record of the original 
dimensions. 

How Index is Used for a Duplicate Order. — To duplicate 
any part of an old order, there is a card index of the produc- 
tion sheet prints that are filed under their order numbers. 
These cards are indexed alphabetically under the customer's 
name which is placed on the card near the top edge. The 
body of the card has columns headed as follows: Order num- 
ber — Name of machine — Size — Machine number — Draw- 
ing number. This card is filled out for each order for that 
particular firm. Therefore, given the customer's name, we 
can, by consulting this index, find the order number under 
which the machine was built, and by referring to the produc- 
tion sheet print for that order number, the drawing numbers 
are found. 

The column for size makes it unnecessary to refer to two 
or three production sheet prints. For instance, if an order 
is received for a set of grate bars, the same as supplied pre- 
viously with a 48-inch boiler for Brown & Co., then Brown & 
Co.'s card is referred to and the 48-inch size is located. In 
another column opposite this size will be found the order 
number, and from the production sheet print for that order 
number we can secure the pattern number and number re- 



DRAFTING-ROOM MANAGEMENT 28 1 

quired. If the size were not marked on the card and if a 
number of boilers had been built for that firm, it might be 
necessary to refer to several production sheet prints before 
finding the one for the 48-inch boiler. 

The "machine number" column is used in case a customer 
sells his machine to someone else, the number being stamped 
on the nameplate of the machine. The " drawing number" 
column gives the assembly drawing number and may save 
time if one wanted only an assembly drawing, but it is pri- 
marily intended for orders such as stacks, smoke connections, 
etc., which require only one drawing. No production sheet is 
made for such orders, a bill of material on the drawing giving 
all information required. 

The original production sheets have a card index with 
alphabetical guide cards, and are indexed under the name of 
the machine, as boiler under B. The production sheets are 
numbered in successive order, as made. The shop drawings 
are indexed alphabetically under the name of the part. These 
drawings are numbered and tiled consecutively as made, and 
are given the suffix A or B. A is the large size (18- by 24-inch) 
and B the small size (9- by 12-inch). The A and B drawings 
are numbered and filed independently of each other. Each 
part of a machine is on a separate card, and the cards are 
rewritten from time to time to keep the parts on the card in 
order of size, the smallest size being at top. 

If the order should be to make a machine to the customer's 
blueprints, these prints are numbered consecutively, starting 
with the number after that given to the last blueprint on the 
previous order, and giving it the suffix C or D, as 125-C. The 
suffix C indicates that the patterns shown on that print are 
our property, and the suffix D indicates the reverse. These 
prints are folded and put in envelopes bearing the same num- 
ber, and are filed away in consecutive order, the C and D 
prints being in separate drawers. The C prints are indexed 
with our own A and B drawings, so that we have on the cards 
a complete list of all sizes of patterns or designs of that par- 
ticular part. The D prints are indexed under the name of 



282 



MECHANICAL DRAWING 



the part. The column for print number on the index card 
is for the number given the print by the customer, and under 
the heading "Name of firm," is the name of the customer or 
owner of the print; these two columns are for purposes of 
ready identification. The foregoing is only a bare outline of 
the system, but it will be sufficient to show its cheapness and 
adaptability to the work required of it. 

Limiting the Volume of the Card Index. — While the card 
index has proven a valuable aid in facilitating the drawing- 



Class Milling Machine Fixtures 

Subdivision Fixtures for parts of Multi< 

spindle Drills 

FIXTURES FOR FEED RACKS 



No. of 
Draw- 
ing 



2716 
3563 
4716 
4719 



Date 
Issued 



6-13-1904 

9-27-1905 

12-30-1905 

12-31-1905 



Drafts- 
man 



Smith 
Leland 
Leland 
Leland 



Description 



For 4-spindle 

drill, If center 

distance 

For 3-spindle 

drill , If center 

distance 

For 4-spindle 

drill, 2| center 

distance 

For 4-spindle 

drill, If center 

distance 



Date 
Superseded 



12-31-1905 



Fig. 4. Index Card intended to reduce Volume of Index 

room work, it is apt to become rather voluminous, however, 
if the business is a growing one, and even though one may 
add all the card index guides possible, dividing the index into 
classes and subdivisions, there will invariably be some sub- 
divisions that will contain more cards than are convenient 
to look through every time a drawing is to be found. For 
this reason a system based upon a principle of classification 
will make the index less voluminous and also permit a saving 



DRAFTING-ROOM MANAGEMENT 283 

of time when looking up a drawing. It has been the usual 
•practice to make one card for each drawing indexed. This 
is not necessary, however, so long as there will always be a 
certain number of drawings of the same kind of tools or articles 
that can conveniently be listed on the same card. The card 
reproduced in Fig. 4 shows plainly the principle employed in 
regard to using the index guides, having first guides for general 
classes, and then for subdivisions. On the third line of the 
card is given the general name of the class of articles for which 
the drawings on this card are made. It will be seen that by 
means of this system the card index can be easily reduced to 
a fraction of its original volume. The average life of a draw- 
ing is rather short, and still, as superseded drawings have often 
to be referred to, it is well to systematize the drawing-room 
so that the superseded drawings are kept on file with the 
regular ones, but marked " superseded," and with the date 
the reissue took place\ In order to save unnecessary delay 
in looking up a drawing, the date when the drawing was super- 
seded should also be marked on the card in the index. With 
the exception of these remarks, the picture of the card will 
explain its purpose, and its general usefulness. 

Record of Blueprints. — In some drafting-rooms, a com- 
plete record is kept of all blueprints which are sent to the 
shop or to the customer. A special card index may be used 
for this purpose, there being one card for each tracing. These 
cards are marked with the drawing numbers and are kept in 
numerical order. When a blueprint is sent to the shop, the 
name of the one receiving it is recorded on the card bearing 
the number of the tracing. In this way, it is possible to deter- 
mine easily where every blueprint of a certain drawing can be 
found. Some system of keeping track of blueprints is es- 
pecially desirable in a manufacturing plant where improve- 
ments and changes in the designs and details are constantly 
being made. For instance, if several hundred blueprints are 
constantly being used in various departments, it is essential 
to know where all the blueprints are, in case changes are to 
be made. 



284 MECHANICAL DRAWING 

The practice in some plants is to provide each department 
with a complete book of blueprints for each type of machine 
manufactured. According to one system, when a change is 
made on a drawing, a new blueprint is made to supersede 
each blueprint in the factory. When a blueprint is issued 
from the drafting-room, a card is filled out. On this card, 
the name of the part is entered, the drawing number, and 
the date the blueprint is delivered to the department which 
received it. When a change is made on the tracing, this card 
index shows where the blueprints are and in the column 
marked "change" is noted the date when the new blueprint 
is delivered and the old one is returned. If for any reason 
it is not necessary to change the blueprint, in some depart- 
ments a check mark, asterisk, or some other symbol is placed 
in the column marked "change," instead of the date, and a 
similar mark is made on the back of the card where the reason 
is noted. For instance, if the drilled hole in a casting is to be 
enlarged from \ inch to % inch, it would not be necessary to 
change the blueprint in the pattern shop, as each department 
has its own blueprint. When a department no longer needs a 
particular set of blueprints, this set is returned to the draft- 
ing-room and the date is marked on the index card under the 
heading "returned." 

When blueprints are sent away to customers or to other 
concerns, a separate card index may be used to show how 
many prints are out, and where they may be found. These 
cards are arranged with the customer's or the firm's name in 
alphabetical order. In order to safeguard against the loss 
of valuable drawings in case of fire, record blueprints should 
be made of all tracings and be kept in a fireproof safe or vault. 
Whenever changes are made on the original tracings, care 
should be taken to see that these record blueprints are also 
changed, so as to be kept up to date. 

Record of Sketches. — Draftsmen and especially machine 
designers often make free-hand sketches, and in conjunction 
with this there may be dimensions or other data which are 
worth keeping for future reference. It is the practice in 



DRAFTING-ROOM MANAGEMENT 285 

some drafting-rooms to draw these sketches with a copying 
ink or a copying pencil, and then reproduce them in a special 
copying book used for this purpose. While these sketches may 
be indexed on the index pages of the copybook, when there are 
several books, a card index for the sketches , is convenient. 
These cards may be arranged according to the name and 
kind of tool or machine or with reference to the names of cus- 
tomers. If a sketch is sent to the shop, it is noted in the 
copybook to enable the sketch to be located, if desired. 

In many drafting-rooms, the draftsmen are provided with 
a notebook in which all estimates and calculations are made, 
but it is often difficult to locate the information or data re- 
quired when one of these books has been used for some time 
and a mass, of calculations has accumulated. A preferable 
method is to place such information and data on separate 
cards which form an individual record for each draftsman. 

General Rules and Instructions. — One of the essential 
features of a good drafting-room system is the adoption of a 
number of general rules such as will be particularly valuable 
for new draftsmen who are not familiar with the various local 
standards, etc., which may have been adopted. The informa- 
tion should cover such points as the sizes of drawings, methods 
of dimensioning, including the adopted method of expressing 
tolerances on drawings, the way various degrees of finish are 
indicated in case this method is in use, information regarding 
the use of cross-sectional lines, lettering, etc. 

In some drafting-rooms, in addition to general instructions, 
what are known as "data sheets" are issued. These sheets 
are intended to give tjie draftsman information and data 
pertaining particularly to the material used in whatever 
products are manufactured. For instance, there may be a 
list of the stock of steel carried in stock, including the sizes, 
shapes, and qualities. These data sheets may also contain 
information regarding stock patterns, examples and explana- 
tions of certain formulas commonly used in the design of the 
company's products, and in general, all data which has a par- 
ticular bearing on the designer's work in that particular plant. 



286 MECHANICAL DRAWING 

Record of Changes on Drawings. — As manufacturers are 
continually improving their products, the drafting-room must 
change its drawing, lists of parts, etc. Under no conditions, 
though, should a drawing be changed by any one unless author- 
ized by the chief draftsman, and then the change must be 
made on the detail, assembly, and list tracings, and all prints 
made from these tracings. Changes are also required in the 
patterns and special tools, where these are affected. A record 
of the changes must be made in a file or book kept for that 
purpose. As a rule, changes are made only before a new 
lot of the product is begun, unless the change is of such a 
nature as to demand immediate attention. For this reason 
the foremen of the different departments, and sometimes the 
men, are provided with books in which they record errors, 
suggested changes, etc., and then send them to the drafting- 
room. 

Before making any changes on a drawing, the draftsman 
should place before him a list of all the places in which the 
change must be made and then check off each place as the 
changes are made. Large changes should be sketched on 
detail paper before they are made on the tracing. Sometimes 
a comparatively simple change, like shortening the over-all 
length of a complicated casting will entail considerable labor. 
This may be avoided by changing only the dimensions and 
either underlining or enclosing them in small heavy circles 
which show that the dimensions are out of scale, so that the 
drawing will not measure correctly. Where the change is 
32 inch or less, the lines need not be altered nor a circle placed 
around the figures. Sometimes when an error has been made 
in the shop, a deviation from the drawings will save a large 
amount of costly labor and material, but the change must be 
recorded for future reference either in making repairs or in 
case some attachment or extra part is to be designed for the 
machine. 

In some drafting-rooms, records of the changes are kept 
on the drawing. This may be done by having two columns, 
headed "Revisions"; one of these shows the location of the 



DRAFTING-ROOM MANAGEMENT 287 

change, the other shows the date. In this case, all changes 
made at one time are given a letter, for example A, which is 
placed in a circle alongside each change. Then the letter 
and a number showing the number of changes that bear that 
letter are entered in the first column and the date is entered 
in the second. 

The method of making records of alterations should be 
standardized, because workmen shift about a great deal, and 
all alterations should be uniform. When several different 
systems of noting alterations are used, it is impossible for a 
man to understand all of them without considerable explana- 
tion, and, of course, no man likes to confess ignorance of a 
drawing if he is used to working with drawings. At one of 
the large automobile plants, when a figure is changed, a lower 
case letter is placed alongside the figure changed. This 
letter is surrounded by a circle on the tracing and the same 
letter occurs again where notation of alterations is made. 
Here the letter is followed with the figure as it originally read, 
followed by the date and initials of the man who made the 
correction. In this manner all changes can be so recorded 
as to enable one to find the dimension as it was prior to the 
change, and also to discover when and who made the change. 
If this system were uniform for all drawings, there would never 
be any confusion in regard to alterations. 

Issuing and Storing Blueprints. — In shops where the 
manufacture of duplicate parts is done on a relatively small 
scale, a single set of blueprints may be issued, this set follow- 
ing the work from one department to another. Such a plan 
would not be suitable for a large plant manufacturing a great 
many duplicate machines at one time, and in such cases the 
different departments are supplied with sets of blueprints so 
that work on a given lot of machines can be carried on at 
the same time throughout the factory. Some manufacturers 
prefer to have all blueprints for a given type or design of ma- 
chine bound together in a book or pack, there being as many 
duplicate books as are required by the different departments. 
The advantage of this method is that a complete set of prints 



288 MECHANICAL DRAWING 

is always at hand and the continual replacement of lost prints 
is not necessary, as is the case where loose prints are used. 

A record of the location of these blueprint books is kept in 
the drafting-room. In some cases, separate prints may be 
used, as, for example, in the screw machine department. 
While most blueprints are unmounted, it is the practice in 
many shops to mount them either on some kind of cardboard, 
wood, or possibly on thin steel plates. A thin material is pref- 
erable because the blueprints require much less storage space. 
Another plan is to issue separate blueprints to the workmen 
and also a complete set of prints bound together for the use of 
the machine shop foreman or superintendent. Blueprints after 
being used are filed away in some shops, whereas, in others, 
they are destroyed. When blueprints are soiled and torn 
considerably while in use, it is preferable to make new ones 
when they are again needed. The extent to which blueprints 
are injured depends, of course, upon the length of time they 
are kept in the shop, and upon the kind of work done. 

In shops manufacturing duplicate machines, the blueprints 
are often stored in the tool store-rooms and are given out in 
exchange for checks, the same as tools. It is the practice in 
one of the large locomotive plants to use blueprints for refer- 
ence purposes instead of the tracings. The object of this 
method is to avoid injuring the original tracings by constant 
handling. An extra blueprint is made of each tracing on 
which there is a large letter R, and also the following note: 
"This is a record print and must be treated as an original 
and returned to vault promptly." 

Systems of Designating Parts by Symbols and Numbers. — 
There are various ways of designating machine and tool parts 
by numbers and symbols, such as are found on mechanical 
drawings. A common method is to identify each type of 
machine by a letter, and then add to the letter a number in- 
dicating the size. For instance, D might represent drilling 
machines, L, lathes, etc., and by combining a letter and num- 
ber, as, for example, L-36, the symbol for a 36-inch lathe 
would be obtained. Serial numbers are also assigned to dif- 



DRAFTING-ROOM MANAGEMENT 289 

ferent parts of the machine; if 25 were the serial number of 
the lead-screw of a 36-inch lathe, the complete symbol -would 
be L-36-25. This same symbol could also be used to identify 
the pattern. The relation between numbers may indicate 
the relation between parts; thus, if 30 is the serial number 
of a shaft, 31 may designate a bushing which is assembled 
on the shaft. When the numbers are arranged in this way, 
they enable the assembler to understand more readily the 
detailed drawings, since they show which parts go together. 

Classification of Parts by Numbers or Symbols. — The 
numbers or symbols assigned to different parts may indicate 
in a general way what the parts are made of or their location 
in a complex mechanism which is divided into several groups 
or sections. Sometimes the size or a number simply indicates 
whether the part it represents is a casting or a steel part. 
For instance, steel parts may be given numbers below 500 and 
castings, numbers above 500. This idea has also been ex- 
panded so that the symbol shows quite definitely the nature 
or purpose of the part it represents. 

A method that has been adopted at a plant manufacturing 
adding typewriters is to indicate by the size of the number 
whether the part is a shaft, a collar, any screw machine prod- 
uct having a hole, a casting, a drop-forging, a punched part, 
a machine screw, etc. Thus, the numbers from o to 9 repre- 
sent shafts; from 10 to 29, studs, pins, screws, and so on. 
Numbers from 90 to 99 are reserved for miscellaneous parts 
not in the general classes mentioned. In addition to these 
numbers, letters are used to show to what general group or 
part of the mechanism any detail belongs. For instance, A 
represents the accumulator mechanism; B, the base frames 
and certain other parts; C, the carriage, etc. According to 
this system, if the symbol A-6 were used, this would show 
that the part represented was a shaft of the accumulator 
mechanism. When more than one operation is required, a 
number may be added to the symbol to show the number of 
the operation; thus, if numbers between 50 and 74 represent 
punched parts, then the symbol A-52-2 on the drawing of 



290 MECHANICAL DRAWING 

a die shows that this die is used for the second operation on 
a punched part for the accumulator mechanism. This system 
with more or less modification could be applied to other lines 
of manufacture. 

Lists of Parts. — What are known as " lists of parts" or 
" bills of material" are frequently used in conjunction with 
mechanical drawings. The part list shows what parts or 
materials are needed in the construction of a mechanism and 
serves as a record for future reference. This list is sometimes 
placed right on the drawing, or it may be on one or more 
separate sheets. These sheets are usually typewritten or 
printed forms that are filled in, or the lists are traced and 
are rilled in on blueprints of these tracings. As various classes 
of materials are used in machine construction, such as castings, 
forgings, parts turned from bar stock, and parts purchased 
outside, such as machine screws, oil-cups, etc., the part lists 
are often classified, particularly if a great many parts are 
needed. For instance, there may be one part list for all of 
the castings required, a second list for parts made from bar 
stock, and still another list for purchased parts, such as ma- 
chine screws, bolts, oil-cups, etc. A typical list of parts gives 
the name of the machine or mechanism for which the parts 
are required, the size, and whatever numbers or symbols may 
be needed for identifying it. In addition there are the name 
and number of each part and whatever information is needed 
in ordering, or to form a complete record. For instance, on a 
list of bar stock parts, the information would include the 
number wanted, the kind of steel or other material, the total 
length, and the form of the section. 

These part lists, which are filed for future reference, are 
of great value when another lot of machines is to be con- 
structed, as they show the stock-keeper the kind and number 
of purchased parts needed, and enable the work of ordering 
castings, etc., to proceed without delay and without guesswork 
as to what is actually required. 

Operation Sheets. — When a new mechanical device is to 
be manufactured in a large plant, an operation sheet, or what 



DRAFTING-ROOM MANAGEMENT 29 1 

is sometimes called a general plan, which designates the vari- 
ous operations to be done from start to finish, is sent into the 
shops where the fixtures, gages, jigs, or tools are made. This 
operation sheet is valuable in showing what jigs, fixtures, tools, 
or gages are needed first. Thus each device follows in its 
regular sequence and a jig to be used on the seventieth opera- 
tion will not be made before a jig used on the fifty-eighth opera- 
tion. The equipment and manufacturing methods are clearly- 
brought out before all concerned, and anything causing a 
fluctuation in the daily output can be easily localized and 
readily adjusted. Operation sheets are summaries of each 
standard part of the product, and from them arrangements 
for the manufacture of other devices are often possible at a 
much smaller expense than would otherwise be the case. 

The value of this sheet is increased by placing on it sketches 
of the article with each operation completed. Then the jig 
or fixture maker sees immediately what his fixture or jig is 
desired to accomplish; oftentimes the man who makes the 
working drawings gets incorrect data or fails to put in every 
detail. As these sketches show each successive operation, 
they assist greatly in demonstrating the locating points and 
giving such other data as are often thought unnecessary until 
the jig or fixture is being made. 

Multiple System of Drawing Sizes. — In order to economize 
in tracing cloth and paper, many drafting-rooms have been 
in the habit of using what is known as a " multiple system" 
of sheet sizes. For instance, in one case the largest sized 
tracings and blueprints which are required are 36 by 24 inches, 
and these large sheets form the basis of all smaller sized trac- 
ings and blueprints by a direct method of subdivision, smaller 
sized sheets being 24 by 18, 18 by 12, and 12 by 9 inches. As 
far as the complete using up of tracing cloth and blueprint 
paper is concerned, such a method of cutting up large sheets 
is admirable; but in filing tracings and blueprints, and when 
mailing blueprints in connection with engineering specifica- 
tions, etc., there is an advantage in having a uniform size 
and one conforming to the 8 J- by 11 -inch standard size sheets 






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of business stationery. A practical method of maintaining 
this standard size in the case of large drawings is by a method 
of folding. 

In the plant of S. F. Bowser & Co., Fort Wayne, Ind., blue- 
prints of working drawings are generally made on sheets of 
about the size of the standard business stationery. For small 
shop drawings, blueprints 8 J by 11 inches in size are ample 
to meet all requirements; but in cases where larger sheets are 
necessary, these are made in multiples of the basic size, so 
that they may be folded up and mailed with letters and speci- 
fications, making a neat package which will fit into an ordi- 
nary envelope with the letter unfolded. To facilitate folding 
blueprints to the standard 8§- by 11-inch size, the tracings 
are made with index-marks on them to indicate the points at 
which the prints should be folded. 

Provision for Folding Blueprints. — Figure 5 shows the lay- 
out for different sizes -of tracings that are used and the way 
in which "folding lines" are provided to show how to fold the 
blueprint made from the tracing. It will be seen that the 
sheet size numbers are 1, 2, 3, etc., for sheets measuring 11 
inches from top to bottom, and 12, 13, 14, etc., for sheets 
measuring 2 if inches from top to bottom. For sizes desig- 
nated by numbers higher than 10, the 2, 3, 4, etc., indicate 
the width of the drawing, while the io's digit indicates that 
the height is 2 if inches. For a drawing still larger than any 
of those shown in Fig. 5, the same system of numbering is 
used; for example, a drawing 44! inches wide by 32^ inches 
long would be called size No. 26, the 6 denoting the number 
of folds right to left and the 2 indicating that there are three 
folds from top to bottom. 

How the Folding is Done. — In folding a blueprint, the 
bottom is first folded under, and then, starting at the right- 
hand side, the print is folded under continuously until the 
last fold has been made. The tracings are ruled with a slight 
taper at the top edge of the sheet, starting from the left-hand 
edge; this taper is yq inch per fold, although \ inch is the 
maximum taper which is allowed for the entire length of a 



294 MECHANICAL DRAWING 

sheet. The top edge was tapered in this way so that, when 
the sheet is folded, the corners will not project. After a sheet 
is folded, the corner will be located from Tg" to f inch below 
the top of the outside sheet, thus allowing the folded blue- 
print to be turned over more easily when looking for a print 
in a binder. 

The spacing of the folding lines for different sizes of blue- 
prints is clearly indicated in Fig. 5, and in the case of the 
No. 15 size of sheet, it will be seen, reading from left to right, 
that these lines are spaced 7§, 7J, 7J, 7! and 7 inches, respec- 
tively, while from top to bottom the spacing is 11 and iof 
inches. The reason for having the distance between folding 
lines decrease by regular intervals from left to right and from 
top to bottom is that in folding up a blueprint the bottom of 
the sheet is first folded under, and then, starting at the right- 
hand side, the sheet is folded under until it has been reduced 
to the unit size of 8 J by 11 inches. Decreasing the distances 
between folding lines as indicated makes the sheet fold flat, 
while it would be bulky if the folding lines were uniformly 
spaced. 

Filing the Folded Blueprints. — Another advantage result- 
ing from the application of this system 'of folding prints to a 
uniform size is in maintaining blueprint files in the drafting- 
room, in the offices of the company's engineers, and in any 
other places where a complete set of blueprints is required. 
When the blueprints are folded they are ready for binding in 
the standard loose-leaf binders used for maintaining various 
files of data written on the regular business paper. The way 
in which binding is effected will be most readily understood by 
referring to Fig. 5, which shows the outline of a complete blue- 
print provided with an extension to fit into the binder. At 
the left-hand side of the upper section of the print there is an 
extension one inch in width, which is punched with four holes 
to fit the type EA-3-R loose-leaf binders made by the John 
S. Moore Corporation, of Rochester, N. Y. Should it happen 
that a print is punched badly and has to be repunched, or if 
the blueprint is accidentally torn at the points where the 



DRAFTING-ROOM MANAGEMENT 295 

binder pins pass through the holes, the blueprint does not 
have to be discarded. In such cases, use is made of Denni- 
son's No. 2 gummed rings, which are pasted around the holes, 
thus making the blueprint even stronger than when it was 
new. The arrangement of these guard rings is shown at A. 

Attention has already been called to the practice of starting 
at the right-hand side of the sheet and folding under continu- 
ously. The real object of adopting this method of folding is 
to obtain a folded print which acts like a single leaf when 
mounted in one of the loose-leaf binders. 

The border line, "folding lines," and a space ruled for in- 
sertion of the title, drawing number, etc., are printed on the 
sheets of tracing paper. Particular attention is called to the po- 
sition of the title, drawing number, etc., which are in the upper 
left-hand corner of the blueprint so that when the sheet 
is completely folded up in a binder, this title and drawing 
number appear on each- folded sheet as successive sheets are 
turned over in the binder, without unfolding the sheets. 

Two files of blueprints in binders are maintained in the 
drafting-room, one of these being arranged numerically accord- 
ing to the drawing numbers, while the other has the blue- 
prints classified according to the parts which are shown. In 
the latter file, prints from all drawings of stuffing-boxes, valves, 
pump plungers, etc., are arranged together according to this 
classification, so that, if it is required to locate a drawing of a 
given part without knowing the drawing number, the task is 
greatly simplified by referring to this file. 

Specifying Standard Commercial Parts. — In making out 
bills of material for assembly drawings, difficulty was some- 
times experienced through failure of draftsmen to use the 
proper commercial names in specifying parts which it was their 
intention should be used in assembling. To correct this un- 
desirable condition, each standard commercial part used was 
given a drawing number. In handling this work, the cata- 
logues of all manufacturers from whom such parts as bolts, 
nuts, washers, set-screws, etc., were purchased were gone 
over by the drafting department and lists of the proper com- 



2Q6 MECHANICAL DRAWING 

mercial names of these parts were made with a drawing num- 
ber assigned to each. 

These lists were copied on the typewriter on 8^- by 11 -inch 
sheets of tracing paper with a reversed carbon paper on the 
back, and these made clear blueprints for the use of the drafts- 
men in making up bills of material, so that any given part 
could be specified by its drawing number, thus practically 
avoiding any chance of mistakes being made. These blue- 
prints giving numbers of commercial parts are placed in 
binders and put in the file of blueprints, which is arranged 
numerically, although there is a slight difference in the ar- 
rangement of these prints. The drawing number is the same 
as the part number, which is believed to be a much simpler 
method than having different numbers for the drawing and 
the part. Ordinarily, there is an individual blueprint for each 
number, but in the case of commercial parts, blueprints are 
arranged with twenty numbers on each, in order to economize 
space. 

There are many parts used in the products made by S. F. 
Bowser & Co. which are of the same design and general dimen- 
sions, although one individual dimension may vary for dif- 
ferent cases. For instance, use is frequently made of pieces 
of f-inch wrought-iron pipe which are threaded at both ends, 
and all dimensions of such pieces are the same, except the 
total length. In all such cases, a blueprint of the part is 
made with the variable dimension indicated by a letter, and 
pieces of each different length used are assigned different 
drawing numbers. These blueprints are made up with more 
than one drawing number to a page, with the variable dimen- 
sion tabulated beside the drawing number under a letter 
which indicates this dimension. This practice of putting any- 
where from one to twenty numbers on a sheet is not at vari- 
ance with the idea that the drawing and part numbers are the 
same, because in such cases it is assumed that a single drawing 
has as many drawing numbers as the number of parts shown. 

Record of Assembled Units. — Many pieces are used which 
are termed " partial assemblies" in turning out completed 



DRAFTING-ROOM MANAGEMENT 297 

products at the Bowser plant. These partial assemblies com- 
prise two or more pieces which are made in the factory, as- 
sembled, and placed in stock ready to be drawn out upon the 
requisition of the assembling department. Each of these so- 
called " partial assemblies" is assigned a drawing number, 
just as if it were an individual part, and in making up bills 
of material on assembly drawings of complete products, the 
drawing number of the partial assembly is used as if it were 
a single piece; then, to avoid the possibility of confusion, a 
double star is placed in the column devoted to the specifica- 
tion of the material from which parts are made. In this way, 
the assembling department sees at once that drawing numbers 
with these stars placed beside them are partial assemblies, and 
so there is no danger of confusion. 

Method of Filing Tracings which Eliminates Card Index. — 
For filing tracings where the system of cutting large sheets 
into halves, quarters, etc., is employed for making various 
sizes of tracings, a number of cabinets of different dimensions 
are required if the tracing files are to be kept in orderly fashion. 
The same is true of the different sized tracings required for 
making blueprints according to the 8|- by 11 -inch system of 
sizes which has been described, but a very satisfactory solu- 
tion of the filing problem has been worked out. A majority of 
the tracings of working drawings are of the 8 J- by 11 -inch 
size, and all of these tracings are filed numerically by their 
drawing numbers in standard business letter filing cabinets. 
Cabinets with other sized drawers are provided to take the 
larger tracings, but, in order that their location may be readily 
ascertained, the file of 8|- by 11-inch tracings is made com- 
plete through the insertion of the upper left-hand section cut 
out from blueprints of all other sized tracings, which gives 
the title of the drawing and is filed numerically by the drawing 
number of the tracing. On this section of the blueprint there 
is written with a red pencil the size of the tracing and its loca- 
tion in other sizes of cabinets required for the preservation of 
the larger tracings. Consequently, reference to a single file 
at once shows the exact location of any tracing that is re- 



298 MECHANICAL DRAWING 

quired. This method enables the required tracing to be lo- 
cated in less time than it would take if it were necessary to 
refer first to a card file to ascertain the location of the required 
tracing, and it also makes it unnecessary to maintain such a 
card file. 

Replacement of Obsolete Prints. — In every plant engaged 
in building machinery or in any other line of engineering work, 
it frequently happens that certain minor details of an existing 
design are required to be changed, but frequently the changes 
are such that they may be made on the original tracings with- 
out requiring entirely new tracings to be made. In working 
out the new drafting-room system for the Bowser plant, pro- 
vision has been made for keeping a record of all changes of 
this kind made on old tracings, and also of cases where en- 
tirely new tracings were found necessary. There are certain 
departments in the factory to which new blueprints of changed 
tracings must be sent to replace obsolete prints in the files in 
these departments; and it is the duty of the drafting-room to 
see that such prints are sent out just as soon as they are ready, 
together with a written notification of the change which has 
been made in the design. 

At the time the new print is delivered, the obsolete print 
must be taken by the messenger and returned to the drafting- 
room. Each department to which a new blueprint is sent re- 
ceives the written notification concerning the change in the 
machine detail shown by the print. This work of preparing 
notifications for the different departments to which blueprints 
are sent is handled by a stenographer. A card file is kept on 
5- by 8-inch cards, which constitutes a complete record of all 
blueprints that are sent out to the shop, and of all changes 
in design that are made on tracings. This card file refers 
not only to prints of parts which go into the latest products, 
but also the parts that have been declared obsolete as a result 
of certain changes in design. There is a separate card in the 
file for each drawing, and on the reverse side of the card spaces 
are provided for recording the sending out of blueprints and 
the return of obsolete prints from each department to which 



DRAFTING-ROOM MANAGEMENT 299 

new prints are sent. This guards against any department 
keeping obsolete prints in its files and making parts from 
such prints through mistake. When the print represented 
by a given card in the file is declared obsolete, a small star is 
stamped in the upper right-hand corner of this card and a 
new card is made out for the print in question. Each card is 
marked in its upper right-hand corner with the drawing num- 
ber of the part shown by the blueprint, and it will be apparent 
that there may be several cards of the same number, each 
having the star stamped in the corner with the exception of 
the last card of the given blueprint number, which represents 
the blueprint that shows the part in its latest form. 

On these cards there is a ruled space on which is typewritten 
a statement of the changes made in the design shown by the 
print; and each time a change is made on a design, requiring 
a new card to be made out to record this change, the card is 
marked with the number of changes that have been made in 
the design of the part shown by that blueprint. These cards 
are in sets, there being one for the drafting-room record file 
and one paper sheet for each department which must receive 
a new blueprint and notification of the changes in design of 
the part shown by the print. There is also one sheet of paper 
for the stenographer's use in taking dictation from the chief 
draftsman concerning the changes made in the design, and 
this sheet of paper is ruled with lines like the ordinary stenog- 
rapher's notebook, so that the usual positions above and 
below the line may be used in writing the notations in short- 
hand. After taking dictation concerning the changes in de- 
sign in each blueprint, the stenographer tears off from the 
pad the top sheet on which the stenographic notes were made 
and all sheets on which notifications to department heads will 
be typewritten, together with the drafting-room record card 
on which a typewritten statement of the change in the blue- 
print will be recorded. Tearing off the complete set of sheets 
is an easy matter, because the girl simply runs her thumb 
over the edge of the pad and catches the card, which is torn 
off with the paper sheets that are above the card. 



300 MECHANICAL DRAWING 

Carbon paper is placed between each of the lower sheets so 
that all of the notification sheets and the drafting-room record 
card may be slipped into the typewriter and written at one 
time. At the bottom of each of the notification sheets sent 
out to the factory with new blueprints, there is a slip sepa- 
rated from the main sheet by a perforated fine. When the 
new print is delivered to the department, the old print is 
taken out of the file in that department, and a receipt signed 
for the new print on the slip at the bottom of the notification 
sheet. This receipt is then torn off the main sheet and clipped 
to the obsolete print, so that both may be returned to the 
drafting-room office. A record of the delivery of the new print 
and return of the old one is then made on cards in the drafting- 
room file, and the old print and receipt are destroyed. The 
double entry on the back of a drafting-room record card serves 
to indicate that the card refers to an obsolete print, thus 
serving as a check on the small star stamped in the upper 
right-hand corner to indicate the same thing. 

Drawing-room Equipment and Arrangement. — The equip- 
ment and arrangement of a modern drawing-room is shown 
in Fig. 6 as an example. The main room is made large in 
order that there will be sufficient space for the equipment 
without the danger of overcrowding. At the same time the 
room should be well heated and ventilated, and should prefer- 
ably face the north in order to secure the steadier light which 
is always obtained from that direction. It is a well-known 
fact that the light secured from this direction does not cast 
the conflicting shadows due to the different positons of the 
sun during the day that the light obtained from any of the 
other three directions does. The north wall of the building 
should be given up to window space, so that plenty of light 
can be obtained. It is always desirable for right-handed per- 
sons to have the light coming from the left, and with this 
fact in view it is well to arrange the drawing tables B so that 
when the draftsman is at work he faces the east. 

At the east end of the main room, according to the arrange- 
ment illustrated, will be found files C and cases D running the 



DRAFTING-ROOM MANAGEMENT 



301 



full width of the room, in which drawings, tracings, pencil 
sketches and supplies are kept. The files are arranged on 
each side of the supply cases and extend toward the center 
for about two fifths of the width of the room. The supply 
cases D are arranged between the files C and are about half 
their size. The file cases consist of seven sets of files, as 
shown in Fig. 7 at b, c, d, e, /, g and h, respectively. A card 
index, a, which is very valuable to a drafting-room, is arranged 
at the top of the case. The file b is reserved for blueprints of 
parts, and assemblies of standard products of the plant, file c 
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Fig. 6. Plan of a Drafting-room showing an Approved Arrangement 

sketches, and file / is for foreign blueprints, that is, blueprints 
which have been obtained from other companies. The file g 
is for obsolete drawings and prints that are retained for refer- 
ence. There are eight rows of these files and the drawings of 
the different years are placed in separate rows, thus making 
them easily accessible and easy to find. 

The supply case D consists of two sets of drawers in which 
supplies, such as tracing cloth, paper, ink, pencils, erasers, etc., 
are stored. The top of the supply case is constructed of such a 
height that it also answers the purpose of a table, which will 
be found very convenient when consulting any of the draw- 
ings or prints in the files. On the south side of the room is 
found another set of files similar to the one just described. 



3° 2 



MECHANICAL DRAWING 



The sample cases E, in Fig. 6, are arranged at the west 
end of the room. These cases consist of several drawers and 
two or three small cabinets, and are intended as a place to 
store defective castings, patterns, new parts, etc., which are 
used for reference by the draftsmen from time to time. 

One of the principal features of the modern drawing-room 
is the portion set apart for the blueprinting work. This 
room should be of light-proof construction and should be 
equipped with all of the modern improvements which have 
been invented for the process of blueprinting, such as blue- 
printing machines, steam driers, washing pans, etc. The loca- 
tion of the blueprinting machine is at F, the steam drier at G 
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Fig. 7. Arrangement of Files and Supply Cases 

The chief draftsman's or engineer's private office is located 
directly in front of the blueprint room. It is advisable for 
the head of the department to have a room separated from 
the drawing-room, where he can discuss his business problems 
with his employes in private. All of these rooms should be 
large and well ventilated. The roof should be equipped with 
a number of ventilators W, in order that a good circulation 
of air can be obtained. The illustrations shown here are a 
combination of the lay-out and equipment of several of the 
drafting-rooms in use at the present time in a number of the 
most modern plants in this country. 

Drafting-room Lighting. — Few classes of work call for more 
active and constant use of the eye than that of the draftsman. 
The necessity for continual distinction of fine lines and details 
and the use of finely divided measuring scales and delicate 



DRAFTING-ROOM MANAGEMENT 303 

instruments warrants a system of illumination free from all 
features likely to produce eye fatigue and eye strain, and 
capable of promoting ease and comfort in such work. The 
problem is not altogether one of providing light of high inten- 
sity. Too much light may be as harmful as insufficient light. 
The following information on this subject was contained in 
an article in the Electric Journal by C. E. Clewell, a specialist 
on lighting. 

The general requirements for drafting-room lighting are: 
Good and sufficient light for each person; uniform distribution 
of light provided by lamps in such numbers and so arranged as 
to furnish illumination which is satisfactory without regard to 
the arrangement of tables; an arrangement of lamps that will 
avoid glare and subsequent eye strain; a system which will 
furnish illumination on the drawing-boards with a minimum 
of shadow effect when using instruments and ruling devices; 
an intensity of illumination which will permit the discern- 
ment with ease of fine lines and detail, and which will be suffi- 
ciently penetrating for tracing work. 

Prevalent Methods of Lighting. — Numerous methods have 
been used foi the lighting of drafting-rooms, some of which 
possess several of the features outlined above, but they sel- 
dom fulfill all requirements. For example, one method of 
drafting-room illumination is that in which one or two light 
units provided with reflectors are placed close to the work. 
This system, casting an intense light on the paper, is not uni- 
form, however, and it is necessary to change the units when 
the position of desks is shifted, wiring modifications often 
being called for in such cases. A system of this kind pro- 
duces a glare from the surface of certain kinds of paper with 
subsequent eye fatigue. It should be further noted that the 
resulting shadows are excessive and this requires a continual 
shifting of the work or lamps and a consequent delay and 
annoyance. 

In an investigation of drafting-room lighting, tests were 
made in a typical room with bays 16 by 20 feet and a ceiling 
height of 11 feet 6 inches. A sectional view and floor plan of 



3°4 



MECHANICAL DRAWING 



such a bay, together with the lighting arrangement, is shown 
at the left in Fig. 8. This typical drafting-room contained 
an average of four tables per bay and could accommodate four 
persons per bay. The room was originally equipped with 
large light units spaced on an average of from 8 to 10 feet 
apart and mounted 10 feet above the floor or about 5 feet 6 
inches above the drawing-boards. The complaints from the 
use of this lighting scheme were threefold: (1) The illumina- 
tion was not uniform, the intensity on some desks being higher 



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Smaller Lamps 

than on others. (2) The low mounting height of the lamps, 
together with the large size of the units required to furnish 
sufficient light, caused those working in certain positions to 
suffer from excessive eye strain, both from the glare of the 
light source and from the reflected light on the papers. 
(3) Shadows from the small number of light units were dense 
and required a constant shifting of the ruling devices so as to 
receive the light on the work at the proper place. 

The problem was to provide illumination possessing all the 



DRAFTING-ROOM MANAGEMENT 



305 



requirements previously outlined, and with features of such 
excellence as to be satisfactory in all respects for a class of 
work which rightfully calls for superior lighting facilities. 
The study of the requirements will show that uniformity, the 
absence of shadows, and the reduction of glare are the condi- 
tions most difficult to obtain. Several methods were given 
thorough trial before the final scheme was chosen. 

Illumination Experiments. — The first step in an attempt 
to eliminate the defects of the lighting system shown at the 



m 



urn 



lis vis 



5 5. 



I S 




Q D- 






* * 



# * 



■Q 



* * 






Q-— ~- 



Machinery 



Fig. 9. Bay lighted by Four 100-watt and Eight 40-watt Lamps — Arrangement 
finally adopted consisting of Sixteen 40-watt Lamps 

left in Fig. 8 was the installation of nine units somewhat 
smaller than those originally used, arranged as indicated at 
the right in this same illustration. Certain draftsmen were 
set to work in this trial bay. From the beginning the follow- 
ing items were observed: The intensity was excellent, the 
light uniform, and the glare inappreciable. It soon became 
apparent, however, that the shadows cast by the large num- 
ber of units were an objectionable feature. In drawing circles 
and in the use of the divider generally, some nine shadows 



306 MECHANICAL DRAWING 

standing out in all directions from the instrument and ap- 
parently rotating when a circle was described produced con- 
fusion and annoyance. This feature naturally gave rise to 
considerable complaint and led to the suggestion that the 
shadows might be diminished by the use of more units ar- 
ranged in groups for a given floor space. 

As a second experiment, twelve units in four groups were 
arranged as shown at the left in Fig. 9, the system being made 
up of four ioo-watt and eight 40- watt tungsten lamps per bay. 
Draftsmen were then placed in this bay so as to work under 
the light for some days. The same trouble was experienced 
with shadows in excessive numbers, as was the case in the 
first trial, the effect being even more noticeable, due to there 
being twelve lamps per bay instead of nine as before. The 
lack of uniformity was even more noticeable in this scheme, 
since each cluster can be considered as one light source as far 
as independence of desk locations is concerned, and the su- 
periority of nine over four light sources or groups per bay 
was demonstrated. 

Other arrangements which were given trial were as follows: 
One bay was provided as an extreme case with 21 units scat- 
tered over the ceiling. Here the shadow effect was perhaps 
somewhat offset by an excessive intensity, but the use of 
lamps in such numbers would be prohibitive in point of 
economy. 

An arrangement of four 250-watt tungsten lamps per bay, 
equipped with broadly distributing reflectors, was tried. 
While possessing some good points, this arrangement made 
use of units entirely too large for the ceiling height. Calcula- 
tions were made to determine the minimizing of shadow effect 
in large rooms by the use of broadly distributing reflectors 
rather than those of a more concentrating type. This involves 
the building up of intensity at a given point by the light fur- 
nished by many distant light sources rather than depending 
entirely upon the light from one overhead unit. A man lean- 
ing over his work will cast a deep shadow, cutting off nearly 
all of the light if provided by one unit overhead and little or 



DRAFTING-ROOM MANAGEMENT 307 

none from distant units; whereas, if the units are provided 
with broadly distributing reflectors, such shadows will be far 
less noticeable. 

An Approved System of Lighting. — The plan finally adopted 
consisted of the use of sixteen 40-watt tungsten lamps per bay 
arranged in clusters of four each and in an inverted position 
on the fixtures. (See the right-hand diagram, Fig. 9.) The 
primary object in this scheme was the attainment of a light 
free from the shadows found in previous trials. Various 
types of reflectors and fixtures as well as the effective mount- 
ing height of the lamps above the floor were successively 
tried. With the ceiling freshly painted a yellow tint so as to 
present a coefficient of reflection of about 0.7, the following 
items were observed: Opaque reflectors which transmitted 
all the light coming from ceiling reflection, while providing a 
shadowless illumination, did not furnish a sufficient intensity 
for the work in question. The reflectors seemed to give the 
best results when mounted in a vertical position pointing up- 
ward, rather than when mounted in an angular position. 
Reflectors of a softly diffusing quality of glass and furnishing 
a considerable amount of transmitted light to the work, 
seemed to fulfill all the requirements as outlined above. Each 
draftsman, irrespective of desk or table location, received a 
good and sufficient fight. This fight was uniform and was 
made soft and free from glare by a glass reflector, providing 
excellent diffusion and a soft yellow tint. The shadows were 
eliminated and with the use of the correct size of lamp an 
intensity of proper value w T as provided throughout the room. 
This system has been in service long enough to show that, 
within the limitations of ceiling height and types of units and 
reflectors available, a very satisfactory result has been ob- 
tained. It is of interest to note that the number of watts 
per bay with this last scheme was practically the same as 
that found in the original installation. Hence the superior 
results were obtained not by an extravagant installation but 
by a carefully arranged plan of the equivalent number of watts 
in another form. 



308 MECHANICAL DRAWING 

Varying the Intensity of the Light. — In such a lighting 
installation as that just described it is likely that different 
intensities of the artificial light may be needed at different 
portions of the day and evening. At first thought the usual 
conclusion is that more artificial light is required at night than 
on cloudy days. Experience shows the reverse. During the 
day the eye is subjected to a stimulus from daylight intensities 
which are ordinarily many times greater than the intensities of 
artificial light commonly used. In the daytime this causes 
the pupil of the eye to be in a contracted state so that it re- 
quires a greater intensity on the object than is necessary 
when the eye is relaxed as at night. Thus on a cloudy day, 
when the daylight is insufficient, a greater intensity of the 
added artificial light is necessary to produce a satisfactory 
illumination on the working surface than at night. If the 
lighting system has been designed for an intensity suitable 
when used in conjunction with some daylight, it is quite pos- 
sible that the intensity will be too high for comfort at night. 
Some way of changing the intensity of the light without de- 
stroying its uniform distribution is therefore desirable. If 
lamps be turned out here and there at random for the purpose 
of reducing the intensity to the proper value at night, the uni- 
formity of the light is apt to be destroyed. One method of 
varying the intensity, without destroying the uniformity of 
the light, consists in installing the lamps in groups, and turn- 
ing out a part of the lamps in each group. This affords dif- 
ferent intensities without disturbing the uniformity. Often, 
however, the lamps are not in groups. 

The tungsten lamp possesses one feature which can be used 
to advantage in varying the intensity, whatever the number 
and arrangement of lamps. From the normal voltage of the 
lamp to about fifteen or twenty per cent below normal, the 
light of the tungsten lamp maintains its characteristic white 
color. The voltage on such a lighting system may be reduced 
by means of a transformer arranged with a number of second- 
ary taps to give voltages below normal, thus permitting a 
change from normal intensity to lower values without notice- 



DRAFTING-ROOM MANAGEMENT 



309 



ably affecting the white quality of the light. This scheme 
has been used and furnishes a convenient method of varying 
the light intensity without destroying the uniformity of 
distribution. 

Adjustable Drop-light and Reflector. — The most common 
system of artificial illumination in drafting-rooms is by means 
of drop-lights and reflectors, either 16- or 32-candlepower 
lamps being used. One 16-candlepower lamp scarcely gives 
sufficient light for the usual size of drawing-board, and a 32- 
candlepower lamp gives a light which is too concentrated and 
glaring, thus causing a reflection from the tracing, which is 




Fig. 10. Light held in Adjustable Bracket 



injurious to the vision. It has been found that two 16-candle- 
power lamps are preferable. One of these should be placed at 
the extreme left and the other a little at the right of the center 
of the board. If one of the lamps casts objectionable shadows, 
it can be turned out temporarily. The adjustable lamp fix- 
ture illustrated in Fig. 10 is designed to eliminate the use of 
two lights. It consists of an arm and reflector mounted upon 
a rod at the back of the drawing-board. There is a universal 
joint at A which permits the lamp to be placed in any position. 
The connection is with a floor socket and there should be suffi- 
cient length of cord to permit the lamp to be adjusted along 
the rod. 



CHAPTER XIII 
SKETCHING AND PERSPECTIVE DRAWING 

Pencil sketches are commonly used by almost everyone 
engaged in mechanical work. The inventor and designer 
frequently use rough sketches in developing new appliances, 
the mental picture being transferred to paper so that it be- 
comes clearer. Sketches assist in developing an idea, and they 
often reveal faults, or show that the idea is entirely imprac- 
ticable. A number of sketches are often made to illustrate 
different solutions of the same problem. During the pre- 
liminary steps in the development of a design, free-hand 
sketches may serve the purpose about as well as accurate 
drawings, and the former are preferable since they can be 
made quite rapidly. 

Another important use of sketches in mechanical work is 
to show the form and arrangement of existing devices, such 
as tools, machine details, or a complete mechanism. The 
original drawings of a machine may have been destroyed by 
fire or lost; if new ones are needed, rough free-hand sketches 
are frequently made first, because it is not convenient for the 
draftsman to make the scale drawings directly from the ma- 
chine. After one or more sketches have been made, all 
important parts of the machine are measured, and these 
measurements are marked on the sketches, which are then 
used by the draftsman as a guide for making a more complete 
and accurate drawing. 

Sketches are used universally to illustrate ideas of a me- 
chanical nature. For instance, whenever there are discus- 
sions on mechanical subjects, a pencil and paper are generally 
required to show by a drawing or sketch what cannot be 

explained clearly by words alone. While the ability to sketch 

310 



SKETCHING 3 1 1 

is essential to the draftsman or designer, at least some skill 
in sketching is also useful to the man in the shop. The ma- 
chinist or toolmake: often relies upon a sketch to illustrate 
some existing tool or mechanism, or as the means of represent- 
ing what is considered, at least, to be a new or improved 
device or method. Drawing has sometimes been referred to as 
the " language of the shop," and inability to draw or sketch 
is often a great handicap to the shop man. 

Experienced machinists and toolmakers usually know more 
or less about mechanical drawing methods owing to the con- 
tinual use of blueprints in the shop. They understand what 
the different views represent, or are able to "read" a draw- 
ing. Practice in the reading of various kinds of drawings 
tends to teach the methods of making mechanical drawings, 
so far as the number of views required and their arrangement 
is concerned; but the actual making of a neat drawing or 
sketch, especially without the use of instruments, requires 
practice and the understanding of a few simple principles. 
It is the purpose of this chapter to give a few hints on sketch- 
ing to shop men who are already familiar with the projection 
method of drawings, and also to draftsmen who find it diffi- 
cult to make satisfactory sketches. It is a mistake to suppose 
that neat sketches can be made only by those having a special 
talent in that direction. Most machinists and toolmakers can 
become fairly proficient in sketching if they are willing to 
practice, and it is important to remember that the amount of 
practice necessary can be reduced greatly if the proper methods 
are followed. 

An Important Principle in Sketching. — The real difference 
between a poor sketch and one that is neatly executed is that 
a poor sketch is composed of crooked fines which are not of 
the right length or do not bear the proper relation to one an- 
other, while in a good sketch the reverse condition obtains. 
Now, most mechanical drawings, whether they represent a 
complicated mechanism or some simple part, consist largely 
of straight horizontal and vertical lines, which vary as to 
length and as to their location relative to one another. The 



312 MECHANICAL DRAWING 

first step, then, in acquiring skill in sketching without the aid 
of instruments, is to learn how to draw fairly straight lines, 
and the next step is to make lines of the right length and place 
them where they belong. That sounds easy and, in fact, it 
is not as difficult as many imagine. 

A finished sketch may appear complex and difficult just 
because it is composed of many different lines. The drawing 
of a few lines would not seem difficult, but when many lines 
are combined to form a complete drawing, the making of 
such a sketch is considered almost impossible by many mechan- 
ics who have never considered themselves handy with a pencil. 
Now the fact is that a sketch which is formed of a mass of 
lines may be drawn almost as easily as a much simpler one, 
except that more time is required. If two straight lines can 
be drawn on a sketching pad, there should be no difficulty in 
drawing four, eight, or any required number of lines, which 
is an important point to bear in mind. It is, in fact, a prin- 
ciple which applies not only to drawing but to building ma- 
chinery and to a thousand and one other activities. If each 
detail of the work is carried on correctly, the completed job, 
whether it be a sketch or a complex machine, will finally be 
the result. The appearance of the completed work may 
prove rather deceptive. It may seem to the inexperienced 
as hopelessly difficult when it is, in reality, merely an 
aggregate of simple details put together in the right 
way. 

Most drawings, as was said before, are formed largely of 
straight lines that are perpendicular to one another. Some 
drawings also have a number of circles or arcs of circles, and 
there may also be lines at various angles to each other, and 
irregular curves. It is evident that one who can draw these 
lines, especially the straight ones, with fair accuracy, without 
using mechanical drawing instruments, has at least a good 
chance of becoming reasonably proficient in the art of sketch- 
ing. Locating and proportioning the lines is another essential 
requirement. These two general features of sketching will 
be dealt with. 



SKETCHING 



313 



Drawing Straight Parallel Lines. — It is preferable when 
sketching by the method to be explained to use a pad having 
a cardboard back to make it rigid. The size of the pad is not 
important, although an 8- by 10-inch size will be found con- 
venient for most sketching. A good exercise with which to 
begin is that of drawing straight parallel lines. An easy 
method of doing this is illustrated in Figs. 1 and 2. The pen- 
cil is guided, as it is drawn across the paper, by one ringer 
which slides along the edge of the pad. Either the little finger 




Fig. 1. Drawing Straight Parallel Lines by guiding Pencil from 
Edges of Sketching Pad 



or the ring finger next to it is used to guide the pencil, depend- 
ing on the distance from the edge of the pad to the line to be 
drawn. Unless the line is very close to the edge — ■ say less 
than one inch — the little finger should be used (see Fig. 2). 
As the little ringer follows the edge of the pad, the ring ringer 
slides lightly over the top surface of the pad and supports 
the hand. By adopting this simple method, lines that are 
practically straight and parallel can be drawn with a little 
practice, and the movement of the pencil can be quite rapid 
without impairing the straightness of the lines; in fact, a 



3*4 



MECHANICAL DRAWING 




Tig. 2. End of Little Finger is held against Edge of Pad so that 
Pencil will follow Straight Line 




Fig. 3. Drawing Straight Line which is located near Center of Pad 



somewhat rapid stroke is preferable. This method will be 
found decidedly superior to the common way of attempting 
to draw a straight line by a series of short follow-up strokes. 



SKETCHING 3 1 5 

After it has become easy to draw straight lines near the 
edge of the pad, the pencil should be moved nearer the center 
as shown in Fig. 3. This exercise is a little more difficult 
because the pencil is farther from the guiding finger. The 
pencil is held at different distances from the edge of the pad 
simply by separating the guiding finger more or less from 
the remainder of the hand. When making a sketch, that 
edge of the pad nearest the line to be drawn should ordinarily 




Fig. 4. Lines drawn at Right Angles and in Different Positions by 
guiding Pencil from the Four Edges of the Pad 

be used for guiding, and in this way the entire surface of the 
pad can be covered, if it is not over eight or ten inches wide. 

The next exercise should be that of drawing lines of various 
lengths at right angles to one another, as illustrated in Fig. 4. 
The pencil is guided by the little finger the same as before, 
and the end of the pad is used as well as the side. These 
exercises should be continued until the drawing of lines by 
this method becomes natural and easy. Simple figures, such 
as squares and rectangles of different sizes, should be drawn 
next to obtain practice in the matter of locating the lines 
properly and getting them somewhere near the right length. 



316 MECHANICAL DRAWING 

This next step is a little more difficult than the first because 
drawing a lot of miscellaneous lines is much easier than making 
a drawing of some definite object. 

How to Proceed when Making a Sketch. — If the shape of 
a casting or forging or the arrangement of some mechanism is 
to be shown by a sketch, the lines which form it must be 
properly proportioned, their length and location agreeing 
approximately with the size and location of the different sur- 
faces or parts which they represent. Success in this branch 
of the work is achieved partly by method and partly by prac- 
tice. Some sense of proportion is also needed, but most 
machinists are good judges of short distances and should be 
able with practice to proportion a sketch accurately enough 
if the right method is adopted. It is important to draw the 
main parts first and then the details, because the proper pro- 
portions and locations are secured more easily in this way. 

In order to illustrate some of the points to be observed 
when sketching and the general method of procedure, one or 
two practical examples will be given. As an illustration, 
suppose a sketch similar to the one shown in the upper part 
of Fig. 5 is to be made. The first thing to decide is how and 
where to begin. It would be possible to start in several dif- 
ferent ways. For example, we could first draw the collar at 
the right-hand end, then the threaded end at the left, and 
finally connect these sections by drawing the long horizontal 
lines, but it is quite evident that this would not be the best 
way to proceed. The proper way would be first to draw a 
center fine, then the principal fines, and finally the smaller 
details. Center lines are not always needed in sketching, 
but as a rule they enable the different parts, even of a simple 
sketch, to be more accurately located relative to each other. 
Take, for example, the sketch of a gas engine crankshaft shown 
in the lower part of Fig. 5. In this way, it is desirable to 
have different bearings approximately in fine. To accomplish 
this, three parallel and equally spaced center fines extending 
lengthwise of the shaft should be drawn first. If these lines 
are drawn fairly straight and are spaced about equally, the 



SKETCHING 3 1 7 

crankshaft can be sketched around them easily. This shaft 
could also be sketched without center lines by first drawing 
lightly three sets of double parallel lines. The two lines in 
the center serve as a guide for aligning the shaft bearings, and 
the outer lines serve to locate the crankpin bearings. The 
shaft is then drawn in heavier lines by beginning at one end 
and finishing each crank successively. 

It is neither necessary nor advisable to guide the pencil 
from the edge of the pad for drawing all straight lines. This 



_ _ 


— 






1 ■ — 








4% 
8* 












:Fh // In J \\ 



Fig. 5. Sketches illustrating Methods of Procedure 

would be an awkward way to draw the numerous short con- 
necting ends which form the smaller details on many sketches. 
The pencil should be guided only when drawing the longer 
lines which form the principal part of the sketch. For instance, 
when sketching the crankshaft, the pencil should be guided 
while drawing the center lines or the light parallel guide lines, 
but the cross-lines are so short in this instance that they could 
doubtless be drawn to better advantage, without attempting 
to steady the hand from the edge of the pad. A sketch of this 
kind could be made easily on cross-sectioned paper, but such 
paper may not always be at hand, and a plain pad is just as 
good for many purposes. The cross-sectioned paper with its 



3i8 



MECHANICAL DRAWING 



guide lines accurately spaced is excellent when it is of especial 
importance to make a sketch accurately to scale. 

Whenever possible, only one view is shown on a sketch, 
even though two views might be considered necessary on a 
working drawing. Many parts are understood to be round 
or of circular cross-section by mechanics, and an end view is 
not needed or, in some cases, an explanatory note may be 




Fig. 6. Sketching Small Details — An Exercise in drawing Lines 
and in proportioning Parts 

added to the sketch to avoid an extra view. If one section 
of a circular piece were of special shape or had a slot cut in it, 
an additional view might be advisable. 

The sketching of small details such as are shown in Fig. 6 
is good practice and should precede the making of larger 
sketches. When starting the sketch it is often advisable to 
draw very light guide lines which serve to block out the prin- 
cipal parts of whatever object is being drawn. Such guide 



SKETCHING 319 

lines, as a rule, will greatly assist in proportioning the sketch 
and make it possible to complete the sketch with little 
or no erasing. When simple details can be drawn readily, 
sketches of larger and more complicated parts should be made. 
The example shown in Fig. 7 represents an engine cross-head 
and one end of a connecting-rod. These sketches illustrate 
the important point that plain straight lines which are parallel 
and at right angles to one another form a large part of most 



Fig. 7. Engine Details — Note how Straight Horizontal and Vertical Lines 
predominate in these Sketches 

sketches. There are doubtless many mechanics who do not 
believe that they could make sketches, even after consider- 
able practice, which would be as well proportioned as those 
shown in Fig. 7; and yet knowing how to draw straight parallel 
lines by guiding the pencil from each of the four edges of the 
pad is practically all that is necessary in making sketches of 
this kind, aside from the ability to judge distances between 
lines and the relative lengths of lines. 



320 MECHANICAL DRAWING 

When lines are at an angle with the sides of the pad and 
the finger cannot be used for guiding the pencil, a good way 
to draw such lines, is to«look at the point on the paper where 
the line is to end and then move the pencil quite rapidly 
toward this point. When drawing lines of this sort, a bold and 
fairly rapid stroke is better than a slow, hesitating movement. 
The eyes should be fixed on the place where the line is to end 
instead of following the pencil point. With a little practice, 
diagonal lines that are quite straight can be made in this way. 

Proportioning the Parts of the Sketch. — When a mechani- 
cal drawing is being made, the length and position of the lines 
depend upon measurements; but when making a sketch of 
some mechanical device or machine part, judgment regarding 
the relative sizes of the different details is needed to secure 
the proper proportions, although, as previously mentioned, 
one's judgment can be greatly aided by proceeding in the 
proper way. As an illustration, assume that a sketch is to 
be made which represents a side elevation of a planer. A 
good way to begin this sketch would be first to draw the long 
lines representing the bed, by guiding the pencil from the 
edge of the pad. The length of the lines would be governed 
entirely by the size of sketch wanted. The vertical lines 
which represent the front side of the housing would then be 
drawn to a height proportional to the length of the bed. That 
is, if the housing of the planer being sketched is judged by 
the eye to be about one half as high as the bed is long, the 
height of the vertical lines should be one half the length of 
the horizontal lines. Other important parts can be propor- 
tioned in the same way. After the large parts have been 
sketched to about the correct proportion by this comparative 
method, it will be found quite easy to draw the details some- 
where near the right size. This idea, as applied to sketches 
generally, may be summarized as follows: Draw the long or 
"foundation" lines first and then it will be comparatively 
easy to sketch in the details. 

There is a decided advantage in making the principal lines 
fairly accurate and straight, not so much to secure neatness 



SKETCHING 



321 



as accuracy of proportion. A drawing can be quite rough 
and "sketchy" and still be a good representation of the part 
drawn, provided the proportion is somewhere nearfy correct. 
This is illustrated by the fact that when a mechanical draw- 
ing which has been made by the use of instruments is traced 
free-hand on transparent paper, the lines may be rough and 
uneven, but the drawing has a good appearance, nevertheless, 
because the proportions are accurate. 

Free-hand sketches should not be relied upon too much, 
particularly when they are used in originating new forms of 




Fig. 8. Drawing a Circle by turning Pad beneath Pencil which is 
pivoted between Knee and Little Finger 

mechanism or designs which are rather complex. As every 
machine designer knows, sketches are often very deceptive. 
Sometimes a certain design or arrangement seems to be en- 
tirely practicable, when seen as a crude sketch, but a drawing 
which is accurate to scale shows clearly that some other plan 
must be adopted. For this reason, any method of sketching 
which tends toward greater accuracy without waste of time 
is desirable. 

Drawing Circles without Instruments. — Thus far nothing 
has been said about drawing circles, and unfortunately some 



3 22 



MECHANICAL DRAWING 




Fig. 9. Joint of Little Finger used as Pivot when drawing the 
Larger Circles 




Fig. 10. Top View of Pad upon which Circle is being drawn — 
Note Accuracy of Circle 



SKETCHING 



3 2 3 



sketches require circular as well as straight lines. It is rather 
difficult to draw, free-hand, what, even in the spirit of 
charity, might be called a circle; but circles can be drawn 
quite satisfactorily by the method illustrated in Figs. 8, 9, 
and 10. The circle is drawn by turning the pad around in- 
stead of the pencil. The pad is supported on the knee, and 
the little finger of the hand holding the pencil, supports the 
hand and should be directly over the knee to act as a pivot 




Fig. 11. Drawing Arc of Large Radius by using Elbow as Pivot 

for the pad. The end of the finger may bear against the pad, 
as shown in Fig. 8, but for the larger circles some prefer to 
form a pivot by bending the finger so that the first joint rests 
upon the pad as in Fig. 9. It is necessary to apply some 
pressure in order to prevent the hand from shifting relative 
to the pad which is turned around by the other hand. Figure 
10 shows how accurately a circle may be drawn by this simple 
method. Circles of different diameters may be drawn by 



324 MECHANICAL DRAWING 

varying the distance between the pencil and the little finger. 
This method of drawing a circle is very convenient, although 
it requires some practice. Quite accurate circles may also be 
drawn by this method upon a single sheet of paper. The 
latter is placed upon a table or board and is slowly turned 
about the point where the little finger holds the paper in con- 
tact with the board. 

When a large circle forms a prominent part of a sketch it 
is better to draw it first, as the other details can then be lo- 
cated and proportioned with reference to the circle. For 




Fig. 12. Sketch of Flange Coupling 

example, when making a sketch of the coupling shown in 
Fig. 12, the circle representing the outside of the coupling 
should be drawn first. A center line is then drawn through 
the circle, and in this way the sectional view is located quite 
accurately in relation to the circle. If an attempt were made 
to draw the center line first and then the circle, it might prove 
difficult to locate the center of the circle exactly on the center 
line when the circle is drawn by the method illustrated in 
Figs. 8 and 9. The sketch of this coupling, as well as the 
other sketches shown in connection with this article, were 
drawn by following the methods described. 



SKETCHING 325 

The gearing sketch shown in Fig. 13 is another example 
intended to illustrate the possibilities of drawing circles with- 
out the use of instruments. A simple sketch of this kind 
might be much less accurate than the one shown and still 
meet all practical requirements, although if a neat and well- 
proportioned sketch can be made without special effort and 
unnecessary waste of time, it is certainly preferable. Besides, 
as pointed out before, a reasonable degree of accuracy is often 



^ 




- ir P.P 




- 
.J 






■ 







Fig. 13. Sketch of Spur Gear and Pinion 

essential. The outer circle or arc representing the outer 
ends of the gear teeth in Fig. 13 does not vary more than 
about -h inch from a true arc on the original sketch, and it 
was drawn by the method previously described. When 
making a sketch of this kind, it is easy to locate the side view 
at the right by simply drawing light projection lines across 
from the end view. 

If an arc of quite large radius is required, the elbow can be 
used as a pivot for the pencil, as demonstrated in Fig. n. 



326 



MECHANICAL DRAWING 



The elbow rests upon a table or desk and the pencil strikes 
an arc having a radius which is about equal to the length of 
the forearm. This method was employed for drawing the 
curved lines which form the body of the jack shown by the 







-. 



Fig. 14. Sketch of Wrenches and Screw Jack — Curved Lines of 
Jack Body are drawn by Method illustrated in Fig. 11 

sketch, Fig. 14. After drawing a center line and lines repre- 
senting the top and base of the jack body, these arcs are 
struck and then it is easy to fill in the smaller details and 
secure a well proportioned sketch. 

Use of Sketches in Tool and Jig Design. — Tool designers 
use sketches to advantage when laying out tools, jigs, or 
fixtures for new work. For instance, in connection with 



PERSPECTIVE DRAWING 327 

turret lathe practice, rough sketches are often made to show 
possible arrangements of the tools in order to determine as 
quickly as possible the most effective method of making what- 
ever part is to be produced. Any special arbor chuck or 
work-holding fixture would also be included in the sketch. In 
many cases, these rapidly drawn sketches, especially if fairly 
accurate, serve the same purpose as accurate scale drawings. 

When developing the design of a jig or fixture, sketches are 
particularly valuable. Often a sketch which is made in a 
few minutes shows that an idea which seemed at first thought 
to be good, is entirely impracticable. On the contrary, if the 
design appears to be satisfactory, a scale drawing showing the 
exact arrangement of the different parts can be made from 
the sketch by a draftsman who has not had the experience or 
training to develop original designs. As the design of many 
special tools and of jigs and fixtures depends upon the shape 
of the work, the preliminary sketches should be made around 
a drawing of the part for which the tool or jig is intended. 
Ink or a colored pencil may be used for this outline drawing 
of the work to secure greater contrast with the sketch of the 
tool or jig itself. It may be advisable to make a fairly accu- 
rate drawing of the work. Sometimes cross-sectioned paper 
is especially useful when developing the designs of new tool 
equipment, in order to insure greater accuracy in sketching. 
The squares on the paper serve as units of measurement and 
the sketch may be drawn almost as accurately as when instru- 
ments are used. 

Perspective Drawings. — Perspective drawings are fre- 
quently used in machinery catalogues, in technical magazines 
or books, and for other purposes when the main object is to 
show a picture of a mechanism rather than an accurate rep- 
resentation of it as seen from different sides. An example of 
a perspective drawing is shown in Fig. 15. This drawing 
shows part of the mechanism of an automatic screw machine. 
It illustrates the arrangement of some of the gearing and 
other parts very clearly, and for this purpose is superior to a 
mechanical drawing. Perspective views of this kind are often 



328 MECHANICAL DRAWING 

used in conjunction with the written description of a mecha- 
nism to represent pictorially the relation between important 
parts. 

The elementary principles governing the construction of 
perspective drawings are illustrated in part by the sketches 
in Fig. 16. If the rectangular shaped object shown were 
observed with the eyes on the same level as the "horizon 
line" ab, the object would appear as shown by sketch A, 











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Machinery 



Fig. 15. Perspective Drawing used to illustrate Part of the Mechanism of an 
Automatic Machine 

but if it were seen from the level indicated by line cd, the 
appearance would change, as shown by sketch B. It will be 
noted that lines on the object which are actually parallel 
gradually converge, because in this case the object is seen at 
an angle; as the parallel lines recede, they naturally appear 
to incline inward or toward each other. If these lines are 
extended on a drawing as shown by the dotted extension lines, 
they will intersect at a given point. Thus one group of hori- 
zontal lines intersect at b in sketch A and the other group 



PERSPECTIVE DRAWING 



329 



intersect at a. These points where the lines meet are known 
as "vanishing points" and their position relative to the object 
depends upon the position of the horizon line and the position 
of the object itself. The object shown in Fig. 16 is inclined 
so that the front edges and sides are not parallel to the " pic- 
ture plane" or the plane of the paper; consequently the lines 
converge toward the respective vanishing points. This vanish- 
ing of lines which recede from the picture plane is the basis of 
perspective drawing. It will be noted that the vertical lines 
are parallel, which is due to the fact that they are parallel 
to the picture plane. The horizontal lines would also be 






^S^-r- 





c-f^ 



Machinery 



Fig. 16. Simple Diagrams illustrating the Principle of Perspective Drawing 



drawn parallel, if they were parallel to the picture plane. It 
is because of this vanishing or receding effect of the lines on 
a perspective drawing that the impression of distance is given 
by such drawings. 

Most perspective drawings, such as are used in machinery 
catalogues, etc., are drawn in such a position that the sur- 
faces of one end, one side, and the top of the object are seen 
and the principal horizontal lines of the drawing vanish at 
two points, as in the case of the simple example shown in 
Fig. 16. Such a drawing is defined as a " two-point perspec- 
tive" to distinguish it from the " one-point perspective" 
which represents an object in such a position that one of the 



330 MECHANICAL DRAWING 

principal groups of horizontal lines is parallel and the lines 
of the other group incline toward the vanishing point. 

All perspective drawings are not made strictly according to 
the rules governing perspective. These modified forms are 
sometimes the result of merely judging the location of certain 
lines, or of the vanishing points; the perspective may also be 
deliberately changed to represent more clearly some detail 
which would otherwise be partly or entirely concealed. 

Method of Making a Perspective Drawing. — The theory 
of perspective is that, if rays or straight lines were extended 
from points on the object to the eye of the observer, and if 
the picture or perspective drawing were interposed in the 
right position, the lines would coincide with points on the 
drawing corresponding to similar points on the object itself. 
This fact will be more apparent by referring to Fig. 17. In 
this illustration, which shows how a perspective drawing is 
made, a plan view of a die-block is drawn at the upper part 
of the illustration, and beneath this plan view there is a line 
A A. The method of making the perspective drawing will 
doubtless be clearer, if the student first thinks of this line ^4 ^4 
as the edge of a pane of glass placed perpendicular to the 
plane of the paper, and the point E as representing the eye 
of the observer. 

Now, if straight lines are drawn from each important point 
on the plan view to the "point of sight" E, it is apparent 
that these lines will intersect the flat glass surface. For in- 
stance, line CE intersects it at D; line FE, at G, and so on 
for all the other lines which might be drawn. It is also evi- 
dent that if a perspective view of this die-block were drawn 
upon the flat surface of the glass plate, the edge of which is 
represented by line A A , point C would be located at D on the 
drawing, point F, at G, point H, at 7, etc. When actually 
making a perspective drawing, since the latter is in the same 
plane as the plan view, these points, such as D, I, and G, are 
first located at line A A and are then projected downward, thus 
establishing certain lines and points on the perspective view. 

The first step is to draw line A A and the plan view which 



PERSPECTIVE DRAWING 



331 




332 MECHANICAL DRAWING 

is placed at any desired angle. In this case side JF in the 
plan view is located at 30 degrees from line A A, so that the 
perspective drawing will show the side and end surfaces. 
The line A A is drawn close to the plan view, because if this 
line were considerably below the plan view, the size of the 
perspective drawing would be reduced accordingly. For in- 
stance, with such an arrangement, the distance between the 
intersecting points D and G would be less and the entire per- 
spective drawing would be smaller. The point of sight E is 
next located somewhere below the plan view. It should pref- 
erably be some distance from the plan view, as otherwise the 
perspective will have a distorted appearance. A line is now 
drawn from E parallel to line JF in the plan view and up far 
enough to intersect line A A, Another line is drawn from E 
parallel to line JC, thus establishing another point of inter- 
section on line A A. The horizon line BB is now drawn. 
The location of this line relative to the plan view and to the 
point of sight E depends upon what surfaces are to be shown 
in the perspective drawing. In this case, the horizon line is 
located considerably above the point of sight; consequently, 
the perspective view will show the upper surfaces of the die- 
block. If a view of the bottom surfaces were required, the 
horizon line would be placed below point E. The lines FE, 
CE, HE, etc., are next drawn from the various important 
locating points on the plan view to the point E. These lines 
need not extend beyond line A A, but they should all incline 
toward point E. At the points where these diagonal lines 
intersect A A, vertical lines are extended downward through 
the space where the perspective view is to be drawn. The 
vanishing points V and V\ are located on the horizon line BB 
opposite the points where the lines from point E intersect A A . 
Now that these vanishing points and the vertical lines 
have been located, the perspective may be drawn readily. 
The bottom line K intersects the vertical line extending down- 
ward from corner J and inclines toward the vanishing point 
V\. It will be noted that all the other lines in this group 
also incline toward V\ and terminate at the vertical lines 



PERSPECTIVE DRAWING 333 

which extend downward from corresponding points on the 
plan view. For instance, the line representing one edge of 
the upper surface o* the die-block terminates at L, because the 
vertical line at L is the one extending downward from the 
corner corresponding to point L in the plan view. The ter- 
minating points of the group of lines which incline toward 
vanishing point V are also located with reference to these 
vertical lines projected downward from the plan view. It is 
necessary to determine the vertical location of these inter- 
secting points, such as N and L. The corner J (see plan 
view) lies in the picture plane, assuming that line A A repre- 
sents the edge of this plane; hence, lines in the perspective 
drawing representing the base of the die-block are located 
the true distances apart, as measured along the vertical line 
extending downward from corner /. This is shown by the 
dotted line projected from the end view M, to point N. This 
end view is not absolutely necessary in this case, but it has 
been inserted to illustrate more clearly how a point, such as 
L, is determined. This point L is not the true distance above 
the base line K, because it lies back of the picture plane. To 
locate point L, the true height of the die-block is projected 
from view M to the line extending downward from /, and 
then a diagonal line is drawn to the vanishing point V\. In 
this way, the intersecting point at L is determined. This 
same principle may be applied in locating the lines in a ver- 
tical direction on other perspective drawings. Students 
should practice the making of perspective drawings of different 
parts shown in orthographic projection, beginning with com- 
paratively simple examples. 

Isometric Drawing. — What are known as "isometric" 
drawings are commonly used in preference to ordinary per- 
spective drawings, especially in connection with mechanical 
work. These isometric drawings can be made more easily 
and rapidly than perspective views, and they are used quite 
extensively for such purposes as illustrating the arrangement 
of a system of piping or other lay-outs when it is desirable 
to show in one view all that is necessary for obtaining esti- 



334 



MECHANICAL DRAWING 



mates, or even for the actual work of erection or construction. 
The difference between a perspective and an isometric drawing 
will be seen by comparing the perspective drawing of a die- 
block, Fig. 17, with the isometric drawing of the same die- 
block in Fig. 18. In the isometric drawing, the two groups 
of lines corresponding to the horizontal edges of the object 
do not incline toward vanishing points as in a perspective, 
but are parallel. The parallel lines representing these hori- 
zontal edges are all drawn at an angle of 30 degrees from a 




Fig. 18. Isometric Drawing of the Die-block shown in Perspective in Fig. 17 

true horizontal line, and they incline upward toward either 
the right or the left, depending upon whether they represent 
the end or side of the object. 

As shown in Fig. 18, the end lines incline upward toward 
the left and the side lines, upward toward the right, but they 
are all at an angle of 30 degrees from a horizontal line and 
do not converge as in the case of a perspective. These inclined 
lines and the vertical ones constitute three groups which are 
used to represent the length, width and thickness of the object 
in a single view. Any line belonging to one of these three 



PERSPECTIVE DRAWING 



335 



groups is known as an "isometric line," and all distances on 
these lines are, according to the usual method, laid off to the 
true dimensions, although, strictly, the length should be re- 
duced on account of the foreshortening effect an amount 
equal to a little over 0.8 of the true length. 

In order to facilitate the making of isometric drawings, 
special ruled paper is sometimes used. This paper has, in 
addition to the regular horizontal and vertical cross-section 



_i CAP SCREW- 
2 




Machinery 



Fig. 19. Isometric Drawing of a Pillow-block 

lines, diagonal lines inclining in each direction at an angle of 
30 degrees to the horizontal. Such paper is especially desir- 
able for making free-hand sketches. For example, if a sketch 
of the die-block shown in Fig. 18 were made on an isometrically 
ruled pad, practically all of the lines could either be traced 
directly upon the ruled lines of the pad or be drawn parallel 
to these ruled lines when the latter were not located in exactly 
the right positions. 

Representing Circles on Isometric Drawings. — Circles are 
represented on isometric drawings as ellipses, as illustrated in 



336 



MECHANICAL DRAWING 



Fig. 19, which shows a pillow-block, and also in Fig. 20 which 
shows a shaft coupling partly in section. The ellipses on such 
drawings can be formed quite neatly with a little practice 
when isometric drawings are made free-hand on ruled pads. 
If the drawing is to be made by means of instruments, the 
construction of an ellipse requires more time, although it can 
be drawn easily by the method to be described, which is accu- 



J HOLES 

i DRILL FOR \ U.S.S.BOLT 




Machinery 



Fig. 20. Isometric Drawing of a Shaft Coupling 



be 



rate enough for practical purposes. This method will 
illustrated by taking the coupling drawing as an example. 

The method of procedure is illustrated in Fig. 21. An 
isometric drawing of a square is first made, as shown to the 
right. The length of the sides of this square is equal to the 
diameter of a circle. The large arcs are drawn from centers 
A and B with a radius R and the smaller arcs, from centers 
D and C with a radius r. In this case, two of these diagonal 
lines are horizontal and the other two are drawn with a 60- 



PERSPECTIVE DRAWING 



337 



degree triangle. The relation between this square and the 
drawing of the coupling is shown by the view to the left. It 
will be noted tha'c the diagonal line intersecting centers A and 
B corresponds to the axis of the coupling. In this particular 
illustration, the ellipse represents the outline of a circular 
surface (side of the coupling) which lies in a vertical plane. 
If the circular surface were in a horizontal plane, the same 
method of drawing the ellipse would be employed, although 




Fig. 21. Method of drawing Ellipses on Isometric Drawings 

the square would be in a different position. If Fig. 21 is turned 
around to the right until the axis of the coupling is vertical, 
the circular surface will then be represented in a horizontal 
plane and the relation of the square to the ellipse in this posi- 
tion will be apparent. The diagonal lines of the square will 
now be at an angle of 60 degrees with the horizontal and 
they may be drawn by using a 60-degree triangle. These 
isometric drawings are easily understood and, when properly 
dimensioned, may be used as working drawings, especially for 
representing comparatively simple parts. 



INDEX 



A BBREVIATIONS, used on working 






drawings, 168 



Angles and tapers, designating, 158 
A. S. M. E. standard machine screws, 

table, 207 
Assembly drawings, classes of, 74 

"D ASE-LINE method of dimensioning, 

151 
Beam compass, 36 

Bevel gears, working-drawings of, 230 
Blue- and brown-line prints, 195 
Blueprinting, electric, paper for, 187 
Blueprinting frame, 182 
Blueprinting machines, 185 
Blueprint paper, kinds of, 184 
Blueprints, changes made on, 193 

drying and ironing machine for, 190 

for patternmakers, 92 

issuing and storing, 287 

making, 182 

mounting, 193 

provision for folding, 293 

record of, 283 

restoring over-exposed, 192 
Bolts and screws, 206 
Bolts, machine, 211 

U. S. standard, 212 

U. S. standard square-head, 215 
Bolts, threads, nuts, table of U. S. 

standard, 213 
Bond paper, used instead of tracing 

cloth, 89 
Briggs standard pipe thread, table ; 221 
Bristol board, 61 
Broken sections, 105 
Brown- and blue-line prints, 195 

/^AM curves, for avoiding shocks at 
^"^ high speed, 248 

for harmonic motion, 248 



Cam curves, for uniformly accelerated 
motion, 248 
modification of, for pivoted follower, 
252 
Cams, cylindrical, design of, 246 
designing or laying out, 235 
face, design of, 243 
lay-out for roller on follower, 237 
lay-out when follower is off center, 

239 
lay-out when follower is pivoted, 239 
plate, arranged for positive drive, 244 
plate, having tangential follower, 255 
plate, having uniformly accelerated 

motion, 253 
return, use of, 245 
Cap-screws, 210 
Card index, see "Index" 
Celluloid templets, 46 
Center lines, 82 

Center-to-center dimensions for locat- 
ing holes, 154 
Changes made on blueprints, 193 
Changes on drawings, record of, 286 
Checking drawings, 176 
Checking list, for punch and die draw- 
ings, 179 
Circles, dividing into given number of 
equal parts, 37 
drawn without instruments, 321 
drawn with spring bow instruments, 

32 
drawn with the compass, 34 
instruments for drawing, 31 
Classification of parts by symbols or 

numbers, 289 
Cloth, tracing, 62 
Commercial parts, standard, specifying, 

295 
Compass, 33 



338 



INDEX 



339 



Compass, beam, 36 

Conical shapes, development of, 271 

Conventional methods of representing 

screw threads, 200 

Cost of production, effect on design, 93 

Court decisions as to property rights in 

engineering drawings and data, 97 

Cross-sectional shapes, simple method 

of showing, on drawing, 126 
Cross-section paper, 61 
Cross-slide and angular methods of 

dimensioning, 156 
Curves, cycloidal, drawing, 264 
involute, 262 

involute, drawn accurately, 263 
irregular, 43 
Cycloidal curves, drawing, 264 
Cylinders, intersecting, of unequal 

diameters, 270 
Cylindrical cam, design of, 246 

"P\ESIGNING a new mechanism, 
"^^ preliminary work in, 70 
Designing or laying out cams, 235 
Designs, how originated, 70 
improving after mechanism is con- 
structed, 95 
Detail drawings and general views, 8 
Detail drawings, when required, 76 
Details, important, sectional views of, 

113 
machine, drawings of, 197 
screw thread, 198 
Developer for restoring over-exposed 

blueprints, 192 
Development of an elbow, 267 
Development of conical shapes, 271 
Dimensioned drawings, examples of, 

136 
Dimensioning, angular and cross-slide 
methods, 156 
base-line method of, 151 
drawings according to metric system, 

167 
drawings, improved method of, 145 
drawings, standard rules for, 135 
working drawings, methods of, 130 
Dimension lines, 82 



Dimensions, arrangement of, for locat- 
ing holes, 149 
center-to-center, for locating holes, 

on one drawing for parts of different 
sizes, 165 

unnecessary, 147 
Dividers, 36 

plain, hairspring, and combination, 38 

proportional, 38 
Dotted lines, use of, to show interior or 

concealed surfaces, 19 
Drafting fabric, opaque, 63 
Drafting machine, 50 
Drafting-room lighting, 302 
Drafting-room systems, equipment and 

arrangement, 273 
Drafting tables, 56 
Drafting tools, set of, 48 
Draftsman, difference between designer 
and, 3 

value of practical shop experience, 
94 
Drawing-boards, 53 

parallel attachment for, 55 
Drawing paper, cross-section, 61 

general considerations in selecting. 
62 

sizes and kinds, 58 
Drawing pencils, 66 

sharpening, 67 
Drawing problems, geometrical, 257 
Drawing-room equipment and arrange- 
ment, 300 
Drawings, assembly, classes of, 74 

checking, 176 

filing, general system of, 274 

free-hand, 310 

isometric, $^^ 

of parts for which tolerances are 
specified, 137 

perspective, 327 

procedure in making, 70 

sizes of, 77 

special, for patternmakers, 91 

'standardization of, 181 

tabulated, 161 

uses of different classes of, 73 



340 



INDEX 



Drying and ironing machine for blue- 
prints, 190 

Drying blueprints, after washing, 
method of, 188 

Duplicate orders, index used for, 280 

C^LBOW, development of an, 267 

three-piece, development of, 269 
Electric blueprinting machine, flat-glass 
type, 186 
vertical type, 185 
Ellipse, approximate method of draw- 
ing, 258 
drawing an, 257 

straightedge or trammel method of 
drawing, 259 
Ellipsograph, the, 259 
Epicycloid, the, 265 
Equipment of drafting-rooms, 273 
Erasing pencil and ink lines, 68 
Errors on drawings, why costly, 175 
Extension or reference lines, 83 

UWCE cam, design of a, 243 

Filing drawings, general system of, 
274 
Filing folded blueprints, 294 
Filing tracings, method which elimi- 
nates card index, 297 
Finish marks, 169 

Finish, symbols indicating kind, 170 
First-angle and third-angle projection, 

16 
Fittings for pipes, 221 
Folding blueprints, provision for, 293 
Follower, cam lay-out for roller on, 237 
off center, cam lay-out for, 239 
pivoted, cam lay-out for, 239 
Frame for blueprinting, 182 
Free-hand sketching, 310 
French curves, 43 

/^EARING, drawings of, 224 
^"^ worm-, detail drawing of, 232 
Gears, bevel, working drawings of, 230 
spiral, drawings of, 234 
spur, methods of drawing, 225 
spur, working drawings of, 226 



Geometrical drawing problems, 257 

LJAIRSPRING dividers, plain, and 

combination, 38 
Harmonic motion, cam curves for, 248 
Helix, construction of a, 261 
Hexagon bolts, rounded, 216 
Hypocycloid, the, 266 

ILLUMINATION experiments, 305 
Index, card, based on classes of 
machines, 276 
eliminated by method of filing trac- 
ings, 297 
for jobbing shops, 279 
for locating drawings, 275 
used for duplicate orders, 280 
Index guides, use of, in card index, 

282 
Ink and pencil drawings, 27 
Ink used by draftsmen, 68 
Instruments and materials, 27 
Instruments, drafting, set of, 48 
Intermittent motion, cam designed for, 

241 
Intersecting cylinders of unequal diam- 
eters, 270 
Intersecting surfaces, development of, 

266 
Involute curve, 262 

drawn accurately, 263 
Irregular curves, 43 
Isometric drawings, 333 
representing circles on, 335 

TZ"EYS on drawings, designating, 223 

AYING out a curve for uniformly 
"^ accelerated motion, 251 
Lead and pitch of screw thread, 199 
Lettering, 174 
Lighting, an approved system of, 307 

for drafting-room, 302 

prevalent methods of, 303 
Limits, designating, on drawings, 142 

drawings of parts having, 137 



INDEX 



341 



Lines, different kinds of, 81 

shade, on drawings, 84 
List of parts, 290 
Locating drawings by card index, 275 

A/TACHINE bolts, 211 
^ A Machine details, drawings of, 197 
Machine, drafting, 50 
Machines, blueprinting, 185 
Machine screws, A. S. M. E. standard, 
table, 207 
classification, 207 
Machine screw standards, 206 
Manufacturing problems, relation to 

design, 95 
Materials and instruments, 27 
Mechanical drawings, classification of, 
6 
number and arrangement of views, 

14 
Metric system, drawings dimensioned 

according to, 167 
Mounting blueprints, 193 

on cloth, 194 
Multiple threads, designating, 199 

NEGATIVES, vandyke, used for 

making temporary forms, 196 
Notes, explanatory, on drawings, 171 
Nuts and screws, S. A. E. standard, 216 
Nuts, bolts, threads, table of U. S. 
standard, 213 

/^\BSOLETE prints, replacement of, 

W 2 9 8 

Opaque drafting fabric, 63 

Operation sheets, 290 

Origination of designs, 70 

Orthographic projection, 10 

method, principle of, 13 

simple examples of, 17 

OAPER, for electric blueprinting, 187 
tracing, 62 
used by draftsmen, 58 
Parallel attachment for drawing boards, 

55 
Part lists, 290 



Parts, designating by symbols and num- 
bers, 288 
Patternmakers, special drawings for, 91 
Patterns and templets, development of 

in sheet-metal work, 266 
Pencil drawing, making the, 86 
Pencils, drawing, 66 
Pencil, spring bow, 31 
Pen, ruling, 28 

spring bow, 31 
Perspective drawings, 327 

method of making, 330 
Photostats of drawings, 196 
Pipe fittings, 221 
Pipe threads, 220 
Piping, drawings of, 222 
Plate cams, arranged for positive drive, 
244 

having tangential follower, 255 

having uniformly accelerated motion, 
253 
Printing titles on drawing, 173 
Prints, blue- and brown-line, 195 

obsolete, replacement of, 298 
Profile paper, 62 
Projection, orthographic, 10 

simple examples of, 17 
Projection, third-angle and first-angle, 

16 
Proportional dividers, 38 
Proportioning parts of a sketch, 320 
Protractors, 42 

Punch and die drawings, checking list 
for, 179 

T> EADING mechanical drawings, 121 
Records, of assembled units, 296 
of blueprints, 283 
of changes on drawings, 286 
of drawings, 172 
of sketches, 284 
Ribbed parts, sectional views of, 106 
Rules and instructions, general, for 

drafting-room system, 285 
Rules, standard, for dimensioning draw- 
ings, 135 
Ruling pen, 28 

drawing straight lines with, 30 



342 



INDEX 



C. A. E. standard screws and nuts, 216 

Scale, draftsman's, use of, 78 
Scales used by draftsmen, 47 
Screws and bolts, 206 
Screws and nuts, S. A. E. standard, 216 
Screw thread details, 198 
Screw threads, representation on draw- 
ings, 200 
Sectional views, 98 

of ribbed parts, 106 
Section lines, 103 
Sections, broken, 105 

representing more than one cutting 
plane, 115 

shown without use of section lines, 120 
Set-screws, 217 
Shade lines on drawings, 84 
Sheet-metal pattern drafting, 266 
Sizes of drawings, 77 
Sketches, how to proceed when making, 
3i6 

record of, 284 

use of, in tool and jig design, 326 
Sketching, 310 

Spiral gears, drawings of, 234 
Spur gears, methods of drawing, 225 

stub-tooth, 229 

working drawings of, 226 
Square threads, drawing, 201 
Standardization of drawings, 181 
Standards and drawings of machine de- 
tails, 197 
Storing and issuing blueprints, 287 
Stub-tooth spur gears, 229 
Studs, 219 

Symbols and abbreviations on draw- 
ings, table, 169 
Symbols and numbers, classification of 
parts by, 289 

systems of designating parts by, 288 

HpABLES, drafting, 56 

Tabulated drawings, 161 
Tangential follower for plate cam, 255 
Tapers and angles, designating, 158 
Tapped or threaded holes, representing, 

204 
Templets, celluloid, 46 



Threads, bolts, nuts, table of U. S. 

standard, 213 
Threads, in holes, representing, 204 

multiple, designating, 199 

pipe, 220 

screw, representation on drawings, 200 

square, drawing, 201 
Thumb tacks, 65 

Titles on drawings and records re- 
quired, 172 
Tolerances, and manufacturing methods, 

relation between, 140 
Tolerances, drawings of parts with, 137 
Tool drawings, tabulated, 162 
Tools, individual tracings of, 164 
Tracing cloth, 62 
Tracing paper, 62 
Tracings, making, 87 
Tracings of tools, individual, 164 
Triangles, 40 
T-square, 39 

Typewritten copy, blueprinting from, 
I9S 

T JNIDRAFT, 64 

Uniformly accelerated motion, cam 
curves for, 248 
ellipse method of drawing curve for, 
_ 253 
Uniform motion, designing a cam for, 

235 
U. S. standard bolts, 212 
square-head, 215 

\TANDYKE negatives, temporary 
forms made from, 196 

Vanishing points in perspective draw- 
ing, 329 

Views, number and arrangement of, 14 

\17ASHING, drying, and printing, 

* * apparatus for, 182 
Working drawings, explanatory notes 
on, 171 
instructions on, 168 
methods of dimensioning, 130 
without tolerances, 141 
Worm-gearing, detail drawing of, 232 



